Even without author's approval. Levine says that on his website as well. 4th edition is the last legit one.
Regarding validity, well, it is a great historical book which contains, besides many other useful things, facts about what pocketknife patterns were called back then, generally
and specifically by the manufacturers. It is not a law nor a prescription for anyone how to name their product, of course not. Moreover, the Guide contains examples when manufacturer calls their pattern differently than it "should" be classified by traditional means. A respectable manufacturer might want to refer to mentioned or similar book from time to time to avoid possible embarrassment (and nay, I don't mean anyone in particular here), but at the end of the day it is their right how to call their product, and our right is to respectfully disagree.
I just realized that I've actually seen many straight croissants and nobody around said there's anything wrong with them.