Entering your Apple ID credentials into a third party app that signs into your account on a Mac Mini in a remote server farm and relays your iMessages through said server farm? What could possibly go wrong?
What I don't get is that you can do this right now. I was using iMessage on my Google Pixel 3a XL from May 2020 to February 2021 when I went back to iPhone.
AirMessage enables iMessage on your Android phone, Windows PC, Linux computer, or Chromebook.
airmessage.org
I would suspect that Nothing is trying to do the same, only they control the Mac Minis.
Just do it yourself, use your own 'server' under your control. You don't need the Nothing phone and you can use it on any Android that can run the AirMessage app.
I don't think they will, even if they dislike it. iMessage is legitimately running on Mac machines, they just happen to be on a server farm. I think it's a security issue, but otherwise I don't care. How does it affect you that you think it should be stopped?
You can do this yourself already with a Mac at home.