I think it would be quite rate for something like this to be feasible at all. Decisions like this are set in stone before development starts, and tend to dictate the development direction.

I'm not going to say you're wrong, but it also adds a ton of complications. Presumably they wrote an entire compatibility layer to map from Unity calls to their own engine. That work doesn't need to be done at all if you start from scratch with your own engine and targets.

You are right that they probably couldn't afford to make their own engine originally though, and this presumably lets them not pay royalties on future sales. I wouldn't say it's a good strategy, but if you end up with a boatload of cash and a game with a long tail then it's a good tactic. It's just not something you should plan on doing.