Yes, you are misunderstanding. I don't see how that's not obvious to you. iMessage is tied to an Apple ID. Unregistering your phone number from that Apple ID do

Yes, you are misunderstanding. I don't see how that's not obvious to you. iMessage is tied to an Apple ID. Unregistering your phone number from that Apple ID doesn't turn iMessage off. The fact that you think it does is where you misunderstand. The only way to turn off iMessage from an Apple ID is to turn off iMessage for the devices that are set up to receive those messages. It's literally impossible to send an SMS message to someone's Apple ID and it's not possible to send an iMessage to a phone number that's been unregistered from iMessage. Just because you don't understand how it works doesn't mean Apple is at fault. You cannot delist an Apple ID from iMessage, you can only sign out of iMessage on a receiving device. That's it. Anything else above that is your fault. If you don't want to receive iMessages at all and don't want people sending them to you, sign out of iMessage. It's not that hard.