During the 2011 season the horsecollar rule (rule 9-4-3k) was clarified with the addition of, "The horsecollar foul is enforced as a live-ball foul." The way it was explained to me, this was because of situations where a defender grabs the collar at the defense's 2 yard line, but doesn't actually pull him down until the runner has taken a step or two into the end zone. The ball became dead as soon as it crossed the goal line but in the case of multiple fouls, horsecollars should be treated as a live ball foul with the potential for fouls offsetting.
That's not what we have here though. Unless the intercepting player went out of bounds, THEN was tackled by the collar, I can't see where even using the OLD rule should have resulted in a dead ball foul. If the player went out of bounds, then I'm with White Hat in saying this should have been a dead ball personal foul. In that case, the yardage they marked off was correct. Live ball and dead ball fouls cannot offset and must be marked off in the order they happened. I don't suppose the officials conferenced after he gave the signal before marking it off, did they? If so, someone on the crew might have caught the problem and they just went with the correct enforcement even if the signal was incorrect.