They have a page about federation, in which they explain why they haven’t implemented federation yet.
In short: privacy concerns (e.g. metadata leakage), lack of devs / manpower & resources, difficulty adapting Revolts protocol to a federated one.
I like the project. I think it would be the perfect place for Foss Projects and a good replacement for Discord, but it is in dire need of more contributors and ppl, who know how to properly integrate Federation. I’d love to see Revolt succeed.
Currently I am short on time, but I’d love to contribute to them in the future.
imo matrix is rather a replacement for public Whatsapp groups. It has a different structure compared to Discord. Revolt has the same structure, but is not federated (yet).
Revolt would be a better choice imo, even when they don't have federation yet, due to missing experience and manpower. See their faq about federation. I'd like to see more people contributing to that project.