16.g4 is possible, but it gives more options to Black. He can continue same as in the game with 16…Bc3 and 17…Nf6, but also dive into wild complications (which would be better in this case) with 16…Ng3 17.Re1 h5 or 17…Bd4 18.Kh2 Qh4 and now Bd4 is preventing Qf2.

In general (this game is exception), with similar pawn structures, White will usually prefer Kh2 over g4 (unless there is tactical justification), because it’s safer. With different pieces’ setup, Black can respond to g4 with all kinds of sacrifices on f4 or g4, or anchor Bd4 with tempo (check when King is not on h2). Also, g4 is loosening protection of the f4 pawn, which itself should serve as support to “programmed” e5.

There are many nuances, it still all depends on the particular position. In our game, it might give some tactical opportunities to Black, and White wanted to avoid it with safer Kh2.

Hope it helps

It seems logical, because Nf6 would just cause major havoc on Blacks already deprived development.
19. e5 After that line is crushing also?