In the article it mentions that the service is run by sunbird. Just by reading their FAQ it doesn’t actually sound like they are MITM’ing messages via some mac server somewhere. It actually sounds more plausible to me that they are doing all the magic “on device”. They specifically mention that this won’t work on multiple phones at the same time, that’s what’s tipping me off.

What I suspect is happening is that the phone itself is spoofing an actual iPhone, and connecting to Apple servers as if it is one. Normally you wouldn’t be able to do this, Apple sells the phones, so they know all the serial numbers that should be able to access iMessage, and would be able to block anything that doesn’t report to be a real iPhone. What I think may be happening is that sunbird could be buying up pallets of dead, old, or otherwise unusable iPhones for pennies on the dollar, and using those serial numbers to pretend they were an iPhone from another device (like the nothing phone) directly.

This would make sense with their business model, according to their FAQ they have “no reason to charge money” for their product yet. Buying access to iMessage for a few bucks upfront with no ongoing cost would match up with what they are claiming, and it would be extremely hard for Apple to detect on their end, as they would appear to be all sorts of models, bought at different times, in different places, and signed in by real people.

I want to reiterate that this is pure speculation on my part, it's just a theory. Which this would mean that (in theory) chats could (and would) be E2E encrypted from sender to receiver, ultimately it’s still Nothing/Sunbird’s app, so they could be doing anything with it on device.