Leaving aside the fact that p2p is certainly not feasible in mobile devices, something being p2p or federated does not improve the service's ability to evade censorship.

How would you block a p2p network like TOR? Just ban all the entry points. You can use proxies to access those, but then you can use the same proxies to access a centralized service.

How do you ban a federated network like Matrix or e-mail? Ban all the servers you can find, then you're back to proxies.

Why isn’t P2P chat feasible for mobile devices? Genuinely curious.

Apps on mobile devices are generally unable to maintain network connections while not in focus.

Is this due to the nature of iOS and Android and them not allowing apps to do that, or is there another limitation at play here?

iOS does not allow apps to be kept alive in background at all. On Android you can (mostly) keep an app open in background, but you should not, as it affects battery life.

Of course, another issue is that no device is always online, so you can't base the system entirely on devices communicating with each other peer-to-peer. You want to at least have supernodes. When you do, it effectively becomes a "dynamic federated network".