I thought maybe the ref just saw Grant's leg in front of the ball and assumed it was someone else's leg (kind of like how a 1st base ump doesn't look at the person, just their foot hitting the bag and the sound of the ball in the glove). He threw the flag and had already convinced himself that someone else was offsides so he didn't consider that it could have been the kicker and he was wrong.
Sometimes I wish certain penalties were reviewable.
Sometimes I wish certain penalties were reviewable.