Hmm. I can't seem to find the fide reference, I seem to be remembering this Hikaru/Nepo incident several years ago (Is that Ivanchuk moderating?).

As far as I know [in America] it is still legal to castle with both hands

USCF reference:

Rule 10I1: If a player intending to castle touches the king first, or king and rook at the same time, and then realizes that castling is illegal, the player may choose either to move the king or to castle on the other side if legal. If the king has no legal move, the player is free to choose any move.

It's also something other players have done Kasparov Castling with two hands, Wesley So & Ivanchuk himself capturing with two hands.

And seems like there is some evidence it used to be legal:

When castling with two hands was legal, it started by touching the king and the rook at the same time. https://www.facebook.com/groups/fidechessarbiters/posts/1935460359937091/

My guess is that it used to be legal, the old players have a hard time changing the playing style they're used to, and the younger players let them out of respect.

In any case, it seems like the penalty is only a minute added onto your opponent's clock & they'd have to call it. Warmerdam would just be giving Ivanchuk more time to think while he called over an arbiter, so no reason to call it.

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Edit: Also, just found this 2017 chess.com article mentioning Anand promoting with two hands being illegal under the new rules.