They don't even have to tackle them to get called for a horse collar. They aren't supposed to grab inside the back or side of the collar.
Horse collar rule
There was a third player. Lead blocker, Purdy and pitch man. The lead blocker could have made a poor block on the defender but I think he was actually doing what he should have, getting to the next level. He's the one who might have missed a block.
Bonner would have been the lead blocker. Had he blocked the guy who stretched it out Purdy would have had a lane either inside or outside. Bonner was looking to make a block downfield which is fine but Purdy didn't do anything to put the defender in conflict. He allowed the defender to play both the qb and rb.
I don't think the Maryland db tried to jam Coleman as much as Coleman ran straight into him as if he was going to block then realized he was supposed to run a route.