[Arathorn]

Signal and Threema are proprietary, in that the protocol they speak is vendor-specific and not openly standardised. You are literally locked to that system, and neither of them allow 3rd party clients to connect.

Moreover, Threema's server is closed-source and so completely proprietary - and you could argue that Signal's server is often closed-source too, given years occasionally go by without public code releases.

This is the rationale.

[Sporktacular]

Signal publishes its protocol spec and allows other applications to use it. Not on their network, but again, that's an issue of federation, not openness. The license allows you to modify it, so you could roll out your own implementation. So you are literally not locked into that system and that's not proprietary.

As for Threema, true enough as it's useless without a server. But again, federation isn't a necessary condition for being open.