You're absolutely right about the KMS requirement for kernel drivers being available and the social aspects of that; when I wrote the article I sort of blithely assumed they were, because they have been for a long time (even in limited form like noveau). But KMS certainly wouldn't have happened without open source drivers that could do enough to require being in the kernel, and that was a product of surprising openness on the part of vendors.

I think it's too strong to say UMS was just a mechanism to keep binary drivers out of the kernel. As far as I know, XFree86 was doing UMS from its beginnings in the early 1990s, which was well before graphics vendors were paying attention to Linux or other free Unixes. There were probably a whole host of reasons that XFree86 used UMS, including that it wanted to be portable across the free Unixes (and not need to coordinate releases with any of them).

(I'm the author of the linked-to article.)