I'm not talking about those rolly suitcases but on United, strollers and luggage carts (the kind that fold up) are not allowed in overhead bin. The size had little to do with the rule (in fact, a small metal object could be more dangerous than something bigger that wont go flying). Your link just said that they're allowed. It didn't mention anything about putting them in an overhead bin. People don't notice what's going on above their heads after landing. How many times did I yell for someone to watch out...

That doesn't prevent strollers from going in a closet.

End of discussion.