Yes it is being used as a verb. It's at least common terminology among speedrunners. Many games, including the one in question, have an in-game progress tracke

Yes it is being used as a verb. It's at least common terminology among speedrunners. Many games, including the one in question, have an in-game progress tracker. To "100%" the game is to complete everything necessary to make it reach 100%. For games without such a tracker, the community usually reaches a consensus on what is considered to be 100%.