Coach, if the player's forward progress had been stopped, then the ball becomes dead at that point. Any physical contact that occurs after that is a dead ball foul. If the officials ruled the foul was during a dead ball, then they got the enforcement correct. No untimed down. That there is no question about.
You're not going to see a personal foul call on a live ball tackle because the player was tackled too hard. At least, I've never seen one called. It just isn't going to happen. And I don't believe the rule you cited supports it. JMO.
My guess is that if your guy had made tis tackle instead of being tackled, we wouldn't be having this discussion, would we?
Good luck during the rest of the season.