Yes, it has an open client (which is not what is used for the official ones)
But despite what their owners are saying, it is fully centralized, and it is not federated.
The owners can do anything, including but not limited to changing the protocol, the rules and the license.
The service may at any later date start including features you don't like (e.g. someone 'leaks' russian signals intelligence has 'influenced' some of its design) and you won't be able to move from the service without losing communication with your existing contacts.
And obviously let's not enter into the fake security features. The best that can be said is that they're not worse than Whatscrap. And that's not saying much.
Out of curisity, so what's attracting you to this service, versus, opening an account on any jabber server and using the client that's built-in in _every_ Maemo/Sailfish version released to date?