Arguably, no, Ur-moment is not a “normal” English expression for most people.
However, it really depends on the company you keep whether it is normal or not. That’s because
ur- is indeed a reasonably productive prefix meaning the original version of something in literary and academic registers, and has been such ever since the second half of the 19th century.
It was borrowed from German. You see it in places like urtext edition, just as one example, to indicate the original, unabridged text of some work. I for example have scads of musical scores on my shelves all labelled Urtext. (OED: “Urtext or urtext: an original text; the earliest version.”)
Regarding ur- in general, the OED says that it is a prefix. . . .
representing German (also MHG., OHG.) ur-, denoting ‘primitive, original, earliest,’ as ur-Hamlet, ‑origin, ‑stock, etc. See also Urheimat, Urschleim, Ursprache, Urtext.
G. ursprache (= primitive language) has been freq. used in recent English philological works.
One thing that makes Ur-comment stand out is its capitalization. That’s for the most part of throw-back to its German origin, where all nouns are capitalized. The assimilated version in English is no longer customarily capitalized, as you will see from these OED citations:
- 1864 Max Müller Lect. Sci. Lang. (1871) II. 133 ― The most troublesome of all vowels, the neutral vowel, sometimes called Urvocal, better Unvocal.
- 1889 Jacobs Caxton’s Aesop I. 37 ― Any light he can throw on the Ur-origin of the Fables.
- 1901 Boas Kyd’s Wks. p. xlv, ― The Ur-Hamlet may have contained a number of these borrowings.
- 1926 A. Møller tr. Pedersen’s Israel I. i. 245 ― The word shēm is found in all Semitic languages and belongs to the absolutely certain ur-semitic components.
- 1927 A. H. McNeile Introd. to Study of New Testament iii. 50 ― It was an Ur-Evangelium, a primitive written Gospel, some say in Hebrew, some in Aramaic, on which our Gospels were based.
- 1937 O. Jespersen Analytic Syntax 142 ― Some well-known students of language who even call this [sc. ‘S is P’] the ‘urform’ of sentences.
- 1943 V. Nabokov in Atlantic Monthly May 69/2 ― The dreadful vulgarity, the Ur-Hitlerism of those ludicrous but vicious organisations.
- 1947 Auden Age of Anxiety (1948) ii. 46 ― For Long-Ago has been Ever-After since Ur-Papa gave The Primal Yawn that expressed all things.
- 1949 F. Fergusson Idea of Theater i. 26 ― An enactment of the Ur-Myth of the year‐god.
- 1950 Psychiatry XIII. 168/2 ― The concept of ur-language and ur-symbolism is of particular importance in Freud’s thought.
- 1964 C. S. Lewis Discarded Image iv. 54 ― Plato’s ur-Freudian doctrine of the dream as the expression of a submerged wish.
- 1966 Punch 9 Nov. 718/2 ― Above is Leonardo da Vinci’s design for an ur-tank.
- 1971 Astrophysics & Space Sci. X. 363 (heading) ― Orientation of galaxies and a magnetic ‘urfield’.
- 1977 Listener 31 Mar. 416/1 ― The importance of the folk example which he [sc. Bartók] argued to be one of the ur-sources of music.
- 1979 Listener 14 June 831/1 ― Sir Nikolaus Pevsner’s ur-history, Pioneers of Modern Design.
- 1983 Sunday Tel. 13 Mar. 14/6 ― Russell Hoban is an ur-novelist, a maverick voice that is like no other.
Notice how in the fullness of time, it has lost its initial capital. That’s what makes your Ur-moment stand out for me: not its existence but its archaic capitalization, which some might consider a trifle “precious” or eye-grabbing.
It does often retain its hyphen, albeit not always. There is some potential for ambiguity without it. For example, urgent just means demanding, but an ur-gent might be Adam. 😇
So you can think of an ur-thing (now probably better spelled urthing) as the first one, or the archetype, or the defining moment in this case, the seed that gave rise to everything else.