Don't know if it really backfired - he got plenty of extra attention by this action. Sales of the movie were not going to increase because it would have that sticker saying "best documentary" on it like other movies have in the past. He and his movies are beyond that level now (whether you like them or not isn't an issue.)
There is only ever one spot, if any, for a non-major studio film in that category and Sideways got the vote this time.