Interesting. I just checked and found the information. It sounds like they want to rebuild the whole app and just add one service at a time. They are starting with just iMessage.

https://www.beeper.com/faq#what-is-beeper

Edit: I wonder how long this will last. They aren't using a Mac to log you into your iCloud account with this. They have found a workaround.

"This is now possible because the iMessage protocol and encryption have been reverse engineered by jjtech, a security researcher."

Edit 2: I'm really curious about this. The researcher has some Python that can run anywhere and let you send iMessages. It seems like something the Apple would shutdown, but, on the other hand, I wouldn't expect Beeper to introduce an app that might get disabled.

https://github.com/JJTech0130/pypush

Its a new version they called Mini because it only supports iMessage natively. There is a research who reverse engineered how iMessage works that's how they manage to make it work without an Apple account and just your phone number.