Seems like all their stuff is open source.
To have self-hosted chat services, you either need a niche enough service that you'll never have two parties that would want to talk to each other while being on different servers, or federation. Signal chose the former, so here I am with eight communication apps on my phone.
Maybe the next best thing could be to support multiple servers, like how email clients let you fetch data from more than one email provider, if they're so worried about federation inhibiting their ability to control the ecosystem that they plainly won't go there and hold speeches about how harmful that situation would be. Then we could have self hosting and also Signal wouldn't have to care about federating with my self-hosted server.