I have seen this particular idea come up a lot lately.

Personaly I think Intel's early investment in linux had a lot to do with it. They also sold a compiler and marketed to labs and such which bought chips. So linux compatibility meant a lot to decision makers.

AMD the underdog went more in on Linux compat than NVIDIA. Which may have been a business decsion.

I dunno, maybe the GPL effect was more a market share thing with developers than a copyleft thing.

Nota Bene: I do love copyleft and license all my own projects AGPL