Sure -- overtime is part of the rules of hockey. Rapid and blitz chess aren't part of the rules of standard chess.
Rapid and blitz (and armageddon) were part of the meta-rules of this match, but then your argument only works if you say Magnus is better at the meta-sport called "winning the 2018 World Chess Championship" -- which I'd agree with, but it isn't the same thing as being better at chess.
Probably the best analogy here is association football -- the normal, standard game can end in a draw. During the World Cup and other elimination tournaments, a completely new rule, not part of the normal game, is introduced allowing games to be decided by penalty kicks. Is the fact that Italy beat France in the 2006 World Cup final on penalty kicks evidence that Italy's team was "better at association football" than France's? Many people would argue that it's not.
Overtime is basically meta-rules as well in ice hockey. It can change depending on the game. ex. it differs depending on if its a playoff game or not. And just like the chess championships, it can change from year to year.
I've competed at a high level in a hobby of mine, and it was well known that the winner of a competition was never "the person who is best", it was "the person who is best in the competition environment that day". No matter what rules you have, that will always be true. And that's okay.
Rapid and blitz (and armageddon) were part of the meta-rules of this match, but then your argument only works if you say Magnus is better at the meta-sport called "winning the 2018 World Chess Championship" -- which I'd agree with, but it isn't the same thing as being better at chess.
Probably the best analogy here is association football -- the normal, standard game can end in a draw. During the World Cup and other elimination tournaments, a completely new rule, not part of the normal game, is introduced allowing games to be decided by penalty kicks. Is the fact that Italy beat France in the 2006 World Cup final on penalty kicks evidence that Italy's team was "better at association football" than France's? Many people would argue that it's not.