We need a time machine, some call it judge on outcome, but he lands on head and neck, that's dangerous. Anything that is dangerous is against the laws, hence it's sanctionable.
Do I have some sympathy with the tackler? Yes, there's no one thing that creates this outcome, but the tackle is borderline in timing, height, and arguably appropriate force for player size and age. The sum of all those parts creates a dangerous outcome - especially in youth rugby, so it needs to be sanctioned.
For the record I don't like judging on outcome, but we have had many, many incidents (eg contact in the air) where outcome has led the sanction as opposed to the force / contact point / intention - so here we are, need to be consistent.