Actually, it seems it is open on match days. TFL changed it's mind. They just don't encourage people to use it:
http://www.hackneygazette.co.uk/news/tfl-u-turn-means-hackney-wick-station-will-open-on-west-ham-match-days-1-4577153
Strange that. This review from football ground guide from an Everton fan says he travelled to and from Hackney Wick. Maybe itwas different last season though:
http://www.footballgroundguide.com/leagues/london-stadium-west-ham-united.html#eddie-ritson-everton-22-4-17