It's a real mess.

Everyone uses arms to jump. You could argue the arm folded due to the defenders shoulder, besides which, it didn't make his head any bigger.

The distance the ball was cleared suggests the fist/hand was in contact with the head giving it the full energy of the rebound, on the expected path.

The rules are such a mess you can why it's given, but you can give several reasons why it shouldn't be.

I hope Hearts not only won the league in Saturday, but do it in style by dishing out a good bumming despite playing a team AND match officials desperate to appease the home crowd.