I found nothing to add to or subtract from Liberman's analysis of others' etymology of 'slang'. That nothing was to be expected. As made clear by Liberman's reference to a note from John Sampson that is not readily available to me, the resources he could bring to bear on the question exceeded mine. This 'answer', then, must present only ephemera of passing interest to me, and possibly others.
No dispute over Liberman's etymology of 'slang' is possible. As he states outright, the etymology is not his. So, it is easy to understand Liberman's frustration, as expressed in his 2016 OUPblog article "The origin of the word 'slang' is known!":
On several websites I now find mentions of the etymology ascribed to me (though I am not its discoverer), but they are brief and noncommittal. At this rate of going, we’ll never make progress. Inspired by the belief that the explanation I defend is not just one of many to be considered but the true one, I decided to return to the question that no longer interests me but may interest some other people. And I’d like to ask: "Why are people so cautious? Why do they hedge instead of celebrating a small victory?" Perhaps because sitting on the fence (or on the hedge, or wherever) is safe, while defending an opinion that has not yet become common property is risky. I would not have fought with such vigor for my own conclusion, but it is not mine, and I feel obliged to break a lance for those who can no longer do so themselves.
For my part, I find Liberman's analysis of the etymology of 'slang' convincing...until and unless a better explanation comes along. However, I disagree with his conjectures about why people are "so cautious". My own "hedge" ("until and unless") is motivated by nothing more than a refusal to firmly and finally conclude any thing whatsoever while a bit of mystery remains; that is, until all the evidence is in and can be examined by me, I'm happy with the etymology recapped by Liberman.
Green's summary dismissal of the evidence as not "concrete", as not "hard evidence" supporting the existing etymology, does contain what he considers the substance of the "relevant note by John Sampson" mentioned by Liberman:
[John Sampson's article] stresses the absence of any links to Romani, while noting the importance of a slang or strip of land....
Among others, notably Hotten (A Dictionary of Modern Slang, Cant, and Vulgar Words, 1860; The Slang Dictionary, 1865), one early secondary source, Barrere and Leland, does ascribe some senses of 'slang' to a "gypsy origin" (see A dictionary of slang, jargon & cant, 1889-90), yet Sampson, an acknowledged expert on gypsy dialect, later contradicted that origin. Barrere and Leland as well as Hotten, however, did contribute a substantial number of the facts later woven together, along with Sampson's denial of a gypsy origin, by Liberman; other facts were contributed by Joseph Wright in his English Dialect Dictionary (1898-1905).
Overall, Liberman's reasoning is easy to follow, and comprehensive. He deserves credit for bringing together the existing evidence and weaving it into a cohesive, logical account of the etymology of slang. That the account has not been more widely accepted rightly merits mild censure.
As always, I exhausted the primary sources available to me before presuming to agree or disagree with the existing origin stories. While largely unproductive, my search did turn up some uses the sense of which remains opaque. For example, the earliest use I found (1724) with reference to a type of language (poetical), did not have sufficient context for me to take the sense with any certainty:
What I have now said on the Topics of Logic and Rhetoric is meant, I own, with a View to what I hope to say hereafter concerning Poetry, properly so call'd. Fain would I, methinks, if possible, retrieve in some measure that Divine Faculty from the stupid Indifferency and Neglect it languishes under in this Age; and get it at least a little of the Esteem it has always before possess'd, in Ages and Countries where Vertue and good Taste have obtained. This were the means, indeed to recommend whatever is Great and Good most powerfully. And if this Attempt of mine could but have any Success; and could but revive, in the polite World, the pure and chaste Spirit of this Gift, instead of the Trade and Slango of it, I should have my End; and feel an inward Pride in helping to raise to its intrinsic Value, in Men's Estimation, an Art, which the Ignorance and Folly, both of many who have pretended to it, and of Others around them, have sunk so very low.
Caledonian Mercury, 06 October 1724 (paywalled)
My best interpretation of "Slango" is something like '[poetry's appeal to] the common taste for vulgar dash, that is, sustained rhythmic jingo ['jingo' in the sense of a "conjuror's call for the appearance of something" (OED)]'.
The uses of 'slang' that I found from before 1724 (having restricted my search to the 18th century) appeared to be uses as the preterite of 'sling', that is, to "move with some force or speed; to fly as if thrown by a sling" (OED) or to "advance, walk, etc., with long or swinging strides" (OED) in the 1703 citation, and the obvious to "throw or cast (stones, etc.) by means of a sling" (OED) in the 1718 citation:
...but know thou, the Emperor was aided by the nearest kinsman he and you have, wherewith she slang away so swiftly, that she seemed to outstrip the wind, and suddenly vanished out of sight.
The Honour of Chivalry, Jerónimo Fernández, 1703
At this Hejage stuck the corner of his Vest into his Girdle, and putting one of the Stones into it that they used to throw out of the Engines, slang it; his Example set them on Work afresh.
The History of the Saracens, Simon Ockley, 1718
After 1724, but prior to the 1756 OED attestation (which they note may refer to "customs or habits rather than language"), I find 'slang' in the sense of "a long narrow strip of land" (OED),
...accessible only by a narrow Slang of Earth which rises like a Causeway in the middle of the Water....
The General History of China, J.-B. Du Halde, 1739
as the preterite of 'sling' in the sense of "throw or cast [with] a sling" in a life of King David (1740), and a use in a letter dated 1595 but published in 1746, the sense of which I have not as yet deciphered, but which seems unlikely to have any connection with the general sense of 'a type of language':
My Lord Monjoy was here, and spake with the Queen in privat, having his Gown girt about hym, because of an Ague: Some say he would faine be a Cownsailor. And I hard that 200 should in privat say to another, whom he trusted much, that the Long Slang, meaning 1000 [Earl of Essex] since d d going away, was never from 1500; and that . . . . now wold doe nothing but what he and my Lord knew.
Letters and Memorials of State