Mastery Points for a Champ I don't play/ don't even own

I have champion points on my account for Thresh - while I have played Thresh, I never played him in Normals. I don't even own this champion on this account, nor have I ever used him in normals.

I would suspect my account was being used by someone else, except this is literally the only evidence of that- match history and wins/losses shows no games played that were not by me and I checked entire match history on Lolking also. I can only assume this is a bug.

Thresh was free about 2 weeks ago, and I played him once or twice in Co Op vs AI, not normals, but this bug appeared after that very recently.

Possible explanation: I played against Thresh in a lot of normal games, when I was Alistar, does this count toward Mastery Points?