This is extremely unethical to offer as a service. Apple account credentials have to be stored as plain text to continuously reauthenticate with apple's servers and the broken e2e means they can store whatever they want from your messages (as well as all other parts of your apple account, like photos and icloud backups).

If they wanted to do this properly the answer is to reverse engineer whatever protocol imessage uses under the hood and make their own client - this is most likely just a simple rebrand of the few FOSS tools that already do the same imessage relay thing.

And of course, when apple inevitable does fight this, the average person will chimp out thinking it's apple doing apple things when in reality no other company would allow a third party to harvest their user's credentials and data in this manner.