The full app, Beeper Cloud, has a bunch of clients built in, that they'll be adding to this (as said in the link). This isn't just iMessage support, it's (eventually) multi-client.

> Over time, we will be adding all networks that Beeper supports into Beeper Mini, including SMS/RCS, WhatsApp, Messenger, Signal, Telegram, Instagram, Twitter, Slack, Discord, Google Chat and Linkedin.