Why did Hikaru take the pawn and put both kings in the middle of the board at the end of this game?

Kings on dark squares means black won. Kings on light squares means white won (always using the 4 center squares for this). Sometimes you see the arbiter place the kings this way after the players leave the board. So doing it immediately after the game ends saves the arbiter the trouble... although maybe also a low key way of rubbing it in as if Hikaru said "black wins" after his opponent resigned.

One king on white, and other other on dark means a draw.

I figured there had to be some reason behind it other than it just being part of the game, since Hikaru doesn't always do that. He was probably still pissed by the arbiter being called over because he accidentally knocked his rook over.

I didn't watch the video, but if they were using DGT board, that is how it tells the software the score and that the game is over.

OTB uses DGT.

Sure, but @martin_stahl wasn't disagreeing with me.

Your first answer was sufficient as it's a long standing tradition before DGT was even invented.