If I understood correctly the Sunbird solution is basically setting up an account on a virtual computer and then relaying imessages? That has all kinds of red flags associated if true.
The obsession with blue bubbles is really silly from all sides of it. Ironically I think Apple intended it to allow people to understand when a message was secure or not, I don’t think they thought it would become a social status marker. Don’t get me wrong, they could have put effort in to better integrate once this all started and clearly they’re leveraging the situation to retain customers - I’m one of them, I gave up and went to iOS because 99% of my family is there and I can’t convince them to adopt Signal, etc.
Having said all of that, trying to build third party middleware at the expense of user security just to fake having an iPhone app is equally silly.