The reference from Etymonline points to both Barre and Debarrer as being active in French around "Late 12c", and for Bar as a noun in English around the same time.
The references for Bar shows English use as a verb back to 13c.
So nearly 3oo years of usage of the word bar as a conceptual "barrier that can be active or inactive" existed before "Debar" is adopted into English. By the way it is used after adoption it's clear that they understood debar represented some "engagement with the barrier".
Since we don't have anything to cite as the reason that "debar" suddenly made the jump to English the best assumption is that one of the first popular uses of word was made by someone who misunderstood the significance of the prefix.
During the time between 12c and 15c there is evidence of the "dis-" prefix becoming the "standard English way" of negating a word; while French became anchored in the usage of the "de-" prefix.
It would even make sense if they failed to grasp the difference between "debar" (negate the barrier) and "d'bar" (of the bar).
Technically I believe these are "Polysemic Contronyms".
Contronym defining the words that contain contradictory meanings.
Polysemic specifying that these multiple meanings are semantically related, coming from the same origin.
Like many examples of contradiction between prefix and meaning, their existence usually represents words borrowed from other languages at different times in their evolution; but ultimately having the same etymological root.