This is actually a great point I didn’t originally consider. People could easily infiltrate the iMessage fort with spam and other stuff which at the moment requires a genuine Apple device.
1: https://developer.apple.com/documentation/sms_and_call_repor...
This explains some things. Why wouldn't they just add a spam filter. Is there still iCloud email addresses? Do they have spam filtering?
but I don't think it's their main reason, if anything I see that argument as convenient posturing which aids in covering the uglier underlying reasons