maybe I just misunderstood
While I can understand that, I don't think it's particularly true. Take Gastrodon, for instance, who just rose from RU to OU. Was it broken in RU? Not at all--it's not that it was overpowered, it's that people realized that it was useful.
There are plenty of pokemon in OU who, if allowed to play in the lower tiers, wouldn't shake up the metagames overmuch.
Similarly, there are plenty of lower-tier pokemon who are perfectly viable in OU. Trying to infer "power" from usage doesn't exactly make sense to me.