Why not 18...Ngf6?

Following up my last post on Planning: Capablanca - Alekhine, St. Petersburg 1913, I read the comments from Chessgames.com against the source game...

Jose Raul Capablanca vs Alexander Alekhine, St Petersburg m (01) 1913
http://www.chessgames.com/perl/chessgame?gid=1011878

...Most comments discussed 'The Greatest Player of All Time', rather than the game itself. One person mentioned 18...Ngf6, but no one picked up the thought. There was also a link to a photo from the game...

Source: Chessbase.com

...which I traced to an article by GM Karsten Müller on Garry Kasparov's Great Predecessors.