The data model obviously supports that. See lines 61-62, you can be in two rooms at once, no problem. In fact, for awhile that's exactly what happened (hence the separation of leave and join).

The only reason I didn't was that the original node.js was usable right from netcat or telnet without prefacing anything with /say or any such command. If I had multiple rooms, a /say <targetroom> <message> would have been required and I didn't think that was very fun to type.