Hmm... You know, I never even considered that. I know that a lot of people can't manage a flat foot, full depth squat, mobility-wise, but I always kind of forget that fact. I've always been able to squat ATG without ever having made any special effort to develop the ability, so I take it for granted.
FTR, I googled the Ido Portal 30/30 squat challenge. You actually don't need to do the full 30 minutes at a stretch, you just need to accumulate it throughout each day, every day. I honestly think Kelly Starrett's recommendation of accumulating 10 minutes a day is a more reasonable jumping off point for most people.