If the client app is open source and the messaging is end-to-end encrypted, I would say it's a lot better than being completely closed. The encryption, which you should now be able to verify, means you don't have to trust the server with your message data.

Though, sure, it would be better for the server software to be open source and rehostable too.

IDK for a messaging app it's not that important to me, because the main value it provides is that service itself to which my network is connected.

Also, the hardest part of such apps is the client component, not the server component. If need arises it shouldn't be hard to build one compatible to the clients.

It looks like they're open sourcing the apps only.