On the broader point though, there certainly are some companies that have access to unique datasets that they can take advantage of. Meta and Google come to mind as obvious ones, probably Microsoft too. Apple I'm less sure given their privacy stance. Any others?
Had a similar experience in middle school. We started initially doing net-send to specific users and having played around a little bit with batch files/command prompt before I tried net send *.
Well the entire county was on the same network. All computers in the school district received my message. Fortunately the message was just "hi", and I think it was only sent once.
Like the OP, in retrospect I saw that the message shows the sending computer+user. Since we had student-specific logins it didn't take long before I was tracked down and reprimanded. I think they even told my parents about it and were threatening suspension etc. But at a certain point it became kind of obvious that the network/configuration was more at fault than a curious kid. So I got off with a stern warning and narrowly avoided a life of crime.