It sounds like a too good to be true situation. Definitely an interesting concept though. Sounds like they use remote servers to connect to the third-party apps using your credentials and then transcribe the messages using the Matrix protocol to the app. Source here and snippet below
Beeper consists of two main components:
- A client app that runs on your devices.
- A web service run by Beeper.
... Beeper’s web service consists of a Matrix homeserver and infrastructure to run open source bridges that connect to 15 different chat networks.
Currently free but also will be a Plus version eventually rolling out, according to the FAQ
For now, everyone has access to all the features of Beeper Plus for free. At some point in 2023, we will begin charging $5-10 per month for Beeper Plus.
Also, no humor is lost on the fact that it is dangerously close to Wuph from The Office...