Well, I do, but this fictional cat platform scenario is kinda non-sense, because it does not anticompetitively interlock multiple business types like Apple does, and has no multipurpose device that serves as a gateway to core technologies used in our economy, infrastructure, and the web.
Anyway, it would be different if this cat platform became heavy weight like Facebook, able to influence citizens(cats) on multiple topics. Yes, you just described Catbook, then yes regulations must apply here, too. That’s exactly what Facebook was at the beginning, just a single purpose platform, but they stretched to multiple business types and must face regulations, just like Apple and Google.