[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.