If there is an iMessage bridge that would be considered a zero-day exploit. There is no official or un-office API for iMessage outside the Apple ecosystem and this is part of security measures by Apple to ensure privacy.
As for the iPhone bridge, that does use a jailbreak which is, of course, an exploit--one that Apple has patched and the patch deliberately not applied to the device in question.
I find this interesting as it raises so many questions:
Do they just give away free iPhones, or reset it for each user and have them mail it back? What if it gets lost / damaged during shipping? How do they cover the costs of this?
What they do is run ichat2json every time there is a new message in a folder and AppleScript to send outgoing messages. It requires a macOS with already authenticated Messages.app.
It's not using any unofficial APIs, it's just a wrapper around iMessage client on a mac.