After reading the Privacy & Security page on their website, it appears this whole discussion of how it may be legal because of a "Mac in the Middle" is irrelevant.
The company claims that the messages are E2E encrypted and no messaging data is stored on their servers. So no Mac.
At Sunbird, we take your privacy and security seriously. Explore our commitment to safeguarding your data and providing a secure messaging experience that you can trust.
www.sunbirdapp.com
So unless Apple has secretly released a public API for iMessage, then we're back to unauthorized computer access. Or fraud if they're lying.