Ivan Sokolov, his former trainer, had this to say about Firouzja:

I think he certainly has a lot of similarities to Magnus, like excellent calculation. On the other hand, just like Magnus, he doesn’t want to enter some crazy Tal-like position or Shirov-like position, sacrificing your house. This is not what he wants.
He’s an active player, with a predominantly active positional style. He plays actively, but on a sound basis. He’s got very good technique, a technique even Magnus felt himself, losing that technical position against him in the World Rapid. Not many people on this planet can beat Magnus in such a technical way.

At the start of the middlegame, the rook may often be a bit slow to enter the play. Pawns or pieces may still be in the way. This can be one of the reasons to advance a rook's pawn. But then sometimes there is another solution: the second rank.

How many of us would, certainly in a rapid game (and this was a rapid game) , automatically reply with the 'standard' 20...Rb8?
However, 20...Rb8? can be met by 21.Nxd6 Nf6 22.Bb3, which immediately creates the annoying possibility of 22.Nf7+.

No worrying now over Nf7+ , anyway, as the rook on a7 is also covering that one!

Here's the point: by using the 7th rank, the rook has sidestepped the knight on d6 and reached an active position on the open e-file, even gaining a tempo by attacking the bishop on e3. Life can be so (deceptively) simple. After...

Black had equalized and the game ended in a draw many moves later.

Of course, White has an overwhelming position, which could easily tempt you to underestimate Black's counterchances. Straightforward, simple development seems to be the obvious thing to do, so 18.Nh3 or 18.Ne2 suggest themselves, connecting the rooks as we are supposed to do. However, this would allow Black the one tempo he needs to contest the e-file with 18...Kg7 followed by ...Re8. Because of this, Gulko played the more precise text move, using the second rank to nip any counterplay in the bud.

It is not clear to me why Black chose this move instead of the natural 18...Kg7 . He was probably already in despair.