> [alas], in american vernacular, and hence also increasingly global, the words [..] have begun to stray...

Except is used more like an interjection there (agreed, except that...). They didn't mean to except the american vernacular from the rest of the sentence, just to clarify why they didn't write "conservative" in the first place.

I read the parent "except" as "the above is true, but..." not as "in all Englishes except American English"