Horse Collar Rule NCAA vs. FED

[NorCalRef12]

The NCAA rules also require that the player must grab the inside of the jersey or inside of the shoulder pads.

"All players are prohibited from grabbing the inside back collar of the
shoulder pads or jersey, or the inside collar of the side of the shoulder
pads or jersey, and immediately pulling the ball carrier down."

Of course, I'm not a college official, nor do I play one on TV. But based on your description I think the R didn't see it clearly and erred on the side of safety.