In terms of chess understanding, does Hikaru know more than Magnus

yes or no?

In terms of chess understanding trash talking, Maurice Ashley knows more than Hikaru and Magnus combined. Hikaru probably knows English better than Magnus so he is better in trash talking. So the answer is yes.
But, regarding chess playing strength, the head to head record between Magnus and Hikaru is very clear!
http://www.chessgames.com/perl/ezsearch.pl?search=Carlsen+vs+Nakamura+
Is there any 2700+ grandmaster who has worse head to head record vs Carlsen compared to Nakamura?

The short answer is no.
Magnus certainly plays better. It also appears imo that he actually understands it slightly better too

The only argument against Magnus is that he seems to be very pragmatic in his play. He avoids super complex positions because he understands boring positions better than anyone in the world.
No one yet is better enough than him at calculation to be able to take him out of his comfort zone without incurring to much of a disadvantage.

It's not about who plays better. It's about chess understanding. That's defined by who is better at interviewing with Maurice Ashley. Magnus is not good at it at all.

There is a difference in playing strength and understanding but i would say Magnus trumps him in both. The only argument I may agree with is that Hikaru is better in certain tactics and perhaps playing unsound attacks.

If Hikaru starts playing at lichess, he will become better.

This is a joke question. Magnus is world #1. Hikaru is world #16. Who plays better?

My dog has more chess understanding than Runekage.

I‘ll make your day. Ready!?

www.chess.com/forum/view/general/nakamura-the-biggest-threat