How was the user able to address me in the chat when I did not even have the reputation to chat and I was not there in the chat ?
I noticed you appearing as well in the Tavern yesterday so I assume you or a rogue script in your browser must have loaded the chatrooms in a tab. Due to your low-rep it showed the 'anonymous' avatar (this was implemented for some sad reasons)
Although you can't speak in the rooms, you can enter them. Users with enough rep can ping any user in the room which was technically the case for you at that moment. And that enabled @ShadowWizard to address you.
You can use the following script to get a list of users in chat:
If you enter a room you would see your username in the list as well.