What fallacy? If BJJ comes from Judo's newaza, and BJJ spends 90%+ of their training on groundwork only...how the hell do they not have a much more evolved and better newaza? Before BJJ, did Judo's 100+ year history know anything about these guards, such as the 50/50, X, Y, Worm, Rubber, etc? Did Judo know anything about the De La Riva pass, Berimbolo, etc? How is this not highly evolved and way more sophisticated? Not all of these techniques transfers to MMA, but enough of them do.
Pure Judo shodans and nidan, when they join our gym and get graded, they usually get a shiny BLUE belt and afterward gets submitted by most of our current Blues. Surely you've seen this as the norm. Now certainly that their standup throws are way better than BJJ'ers, I have no doubt about this. And nowadays, there are plenty of crossovers and hybrid fighters, so I'm just talking about people who only trains Judo.