Except the stakes for Apple are so much higher. If they’ve lied to everyone and are stealing messages, that’s a multi-billion dollar class action, against very little upside to Apple.
For a tiny company like Beeper, the incentives are different. The upside of being dishonest far outweighs the risks.
Not that I believe Beeper is nefarious. They probably aren’t. But their risk/reward for abusing trust is very different from Apple’s
Trust that a third-party application isn't stealing the decrypted messages requires the same type and amount of trust that Apple is not stealing the decrypted messages (or maybe less trust if the third-party solution is open source, etc.).