> Aren't all text messages routed through a server?

For SMS, no not a central server for all carriers. When SMS service originally launched it wasn’t even cross-carrier.

MMS acts like a mailserver at each carrier, sends a push notification and your device has like a day to download full message.

For RCS it’s supposed to be federated but carriers gave up and it’s now centralized through Google Jibe.

I don’t think it’s any more decentralized than Apple if you are talking about MMS (instead of SMS). Your carrier still provides the MMS servers.