Board 4: the move of the match (Byambaa 0 - Schmakel 1)

Is it correct?  I have no clue, but I do know that Black's 18th move is très cool, and shows a desire to fight for the full point.  Normally one does not wish to move pawns in front of one's castled king.  Especially not two squares.  Especially not the g-pawn. Especially not when the players have castled on opposite sides. Especially when one creates a lever that helps the enemy open lines to your king.

But rules of thumb are made to be broken: chess is a very concrete game. The idea is known from the Browne System of the Najdorf.  The e5 square is worth a lot: Black gets a horsie on c4 and a bishop on f6 soon afterwards.  So you could say that Black is attacking White's queenside with 18...g7-g5!?

And Sam Schmakel of Whitney Young must have been planning this shocker some moves earlier: otherwise, why 16...Qd8?