Well, engines made it a different kind of game, so yeah, these days the top players are different when in the past they were also strong OTB players. So different that apparently some of them don't even play OTB. I don't know that you can use that fact to claim they don't know much about chess, just that they focus on that type of chess while OTB pros spend all their energy on OTB chess.
If it's a variant it seems as silly as saying a marathon runner isn't as much of an athlete or doesn't have strong legs because he can't finish top 10 in a bike race.
I would also claim that the main problem for a very strong OTB player to get good at cc would be the time to adjust, and patience, and motivation too I guess. On the other hand, for a B player who is good at cc chess to "adjust" to playing like a GM OTB? I'm sorry there isn't a way lol.
In my other post I mentioned competition, which I still think explains part of this, but this is a good point I think.
Sure, but to be the top OTB player, you compete against (I don't know) 100s of thousands? Among them kids who were raised on chess. I'm guessing ICCF isn't like that.
Plus ICCF isn't about e.g. knowing theory (endgames and openings), so that might not be the best comparison. Maybe an ICCF player could tell you things you wouldn't need to know for OTB play like which openings just don't work, and which are best... of course using a completely different criteria because it's a different kind of game.