This is the point that I think many are missing. Good position isn’t as much about jamming your heels down or sticking your chest out—that’s posing. Good position is similar to the athletic stance you adopt in various sports with a balanced center of gravity. When you throw away the balance for the sake of an aesthetic, that’s not proper position.
So both Karl and McLain may be saying the same thing…but Karl is also on record decrying equitation and dismissing some of the fundamentals. I don’t want to diminish anything he’s accomplished, and I think he’s done a great job of late. But… I think it was Val Renihan in the comments who also pointed out that Karl has a lot of advantages that not everyone has: wealth, top training, and fearlessness. And that Karl struggled for years, and maybe could have moved up the ranks faster by focusing on his own position/technique.
Regardless, for most of us mere mortals, we’re probably better off focusing on our balance and position if we want to ride effectively.
Edited to add: John Madden just posted a very thoughtful take on FB about position and its relation to horse and rider safety. That’s well worth the read IMO