Magnus has played tons and tons of chess games this year and has maintained his rating. There is no doubt his rating is very accurate unless you think his skill started to deteriorate in the last month.
I think your point is fine for Hans, I’m just ruling out the possibility that Magnus is overrated.
The reason a lot of older players are overrated is because they stop playing as often and their rating doesn’t catch up to their decline. Magnus plays a ton so that isn’t a concern.
For elite younger players, no matter how much they play, they are getting better so quickly that the rating can’t catch up.
Younger players also get better much much faster than older players decline. It might take a decade for a GM past their prime to drop 100 ELO, whereas a young player can easily add 100 ELO in a year on their way to super GM status.
