The post is specifically on correcting weaknesses. Leg extensions verboten? That's a blatantly wrong statement because you're using it _absolutely_. Maybe for max loading, but nobody is recommending 1rm leg extensions. Specifically here it's being used to strengthen the VMO, so it would be in a moderate loading range. Without looking it up, can you think of another direct way to improve patellar tracking issues due to overdeveloped vastus lateralis?

Similarly, let's talk about 'functional ab muscles.' Functional is a needlessly vague term. Let's say I'm a powerlifter and I feel my core is limiting my squats. Don't tell me to squat more, because I am an advanced squatter and my core is weak despite heavy squatting, so clearly it would benefit from more direct work. How would you strengthen my abs so my squat can improve? Outside of extremists, most coaches supplement auxiliary lifts to improve weaknesses.

For what it's worth here's a video of Konstantinovs (800+lb deadlift) doing crunches, I don't think he would do that unless he found 'functional' value: https://www.youtube.com/watch?v=vMjXVHHAJ70

I appreciate your enthusiasm but it sounds like you're very clearly a beginner and in situations like this you should keep a more open mind and avoid judging so quickly.