Are they? Casey programmed the walk monster for Jon's game, a brute force tester to make sure you could always walk both ways and never get stuck, instead of building the walking system using a nav mesh. There was one place you could walk where you weren't supposed to, despite the walk monster, that would have been obvious if it was a nav mesh.
John Carmack is a legendary game programmer. If I can identify a superior algorithmic approach to something in Doom, does that disqualify Carmack's credentials?
He’s known for his Handmade hero series (unfortunately archived now) where he developed his game over 500+ episodes: https://hero.handmade.network/
I’m sure you can find bugs in the work of any game dev you would consider legendary, game implementation is generally very messy