It's not even true in chess, centaurs stopped winning against machines unless the human half does nothing and just lets the machine play.

There are centaur matches on the schedule this week for AlphaGo, so that should be interesting.

edit: sources on the chess stuff follow.

http://rybkaforum.net/cgi-bin/rybkaforum/topic_show.pl?tid=3...

http://www.infinitychess.com/Page/Public/Article/DefaultArti...

http://www.infinitychess.com/Page/Public/Article/DefaultArti...