Looks like the client isn’t, but they do offer a simple-way to self-host the backend (looks like it’s “just” a matrix server and a bunch of bridges) and then you can use any open-source matrix client to connect to that. Seems like a pretty good balance of a way to make money and the guts being open enough that one could move if the client/company goes side-ways, while contributing a lot to the open-source community.