"it isn't the WMF legal team who could sue because WMF do not own the text" I guess so.
"at most the chapter by those authors would be released under a free licence" OK, sure.
"they could just offer those two authors $100 to waive their licence terms" nope. Thanks for playing.
I mean, yes, it's technically *possible* that the editors have a copy of the article on their hard drive, and can re-license it to OUP for this purpose. But most likely, they are getting it from Wikipedia, receiving it under CC-BY-SA, and then trying to re-license in a manner incompatible with that license. I'm guessing most editors don't store their work locally just in case some closed publishing house wants to come back later and ask for a re-licensed version.