Their open source code is permissive of commercial forks.
Like, if their goal was to block third-party clients, then they would look more like Facebook or Reddit
They can give you the right to reuse their source code for building your own service using your own network (an aspire to give you this right), but still forbid you to connect to their service on their network with third party clients.
In practice, they don't seem to enforce that a lot (anymore).
I guess they didn't do so for Molly. I suppose a lot of water has passed under the bridge.
But clearly connecting to the Signal servers with an outdated, modified fork is against Signal's goal.