If the Endgame Supremacists were correct then we would see players who primarily studied the endgame from early in their development dominating tournaments at all levels.
But we don't.
I can't think of any player I know personally or from reading who trained that way.
Chess is a very competitive game. If you read chess.com, you know how desperate people are to improve. If endgame study were the magic key, a lot more people would be studying that way and getting results.
I don't understand why people hate him so much. I love his endgame course.