imessage from someone without a phone number or email

So my Brother recently received an imessage to their work phone from a friend, who didn't have their work phone number, their personal number (their personal phone is an android), or their work email.

All they had was my brother's personal Email address (a Gmail) which is not linked with their apple ID in any way.

How is this possible?

My working theory is my brother used to have an apple id and somehow that was connected to his work phone, but he says it's not.