I don't think you get what RoyalDispenser is saying. Tyrunt's hidden ability is Sturdy, but it's not getting Sturdy. It's getting Rock Head. Rock Head has never been a natural or hidden ability for Tyrunt. This is something new that has never been coded into the game before, so there's no telling if the resulting Tyrantrum will have Strong Jaw, Rock Head, or something completely different. If Tyrunt were getting Sturdy, then yes, you could expect Tyrantrum to have Rock Head since they'd both be their respective hidden abilities, but that's not the case here.
EDIT: Greninja'd, which is weird because we just banned it.