Looks like they self-host their source code too: https://framagit.org/framasoft/mobilizon/ It appears to be developed in some language called "Euphoria"?? That's such a strange choice and is going to seriously limit contributions from other developers.
Okay that was a lot of words. Maybe I should explain for those less familiar:
Users on the Fediverse can interact without being aware of the fact that their servers are running completely different software. While Fediverse members such as Mastodon take inspiration from Twitter, others have different focuses: Peertube is YouTube inspired, Pixelfed is Instagram-inspired, and some like Pleroma are kinda doing their own thing imo.
I personally prefer Pleroma to Mastodon, as evidenced from my bio; it's lightweight, easier to set up, and has a great variety of first- and third-party front-ends (including a lightly-modified version of the Mastodon front-end and a Gopher-accessible one).
It appears that Mobilizon devs wanted to create a new Fediverse server with a different focus: something that warranted writing a back-end based on Pleroma instead of just creating a new Pleroma front-end. Basing off Pleroma was a good choice because unlike Mastodon, Pleroma is lightweight enough to run off a singe-board computer like a Raspberry Pi. Being written in Elixir definitely has a lot to do with this.