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?