Before Stratus you could definitely build multiuser chat using FMS, or an open-source alternative like Red5, or many other text-only alternatives such as XMLSocket servers (ElectroServer, SmartFoxServer) or even a custom polling service.
What Status does is allow peer-to-peer connections, rather than relaying data on a server. One of the things that's exciting about peer-to-peer is that you get much lower latency, so making nearly instantaneous realtime interaction is a lot easier.
Flash Player 10.0 allows for multi-user peer-to-peer connections, but Flash Player 10.1 allows for the new NetGroup capibility. NetGroup makes multi-user peer-to-peer easier and more scalable, but it isn't strictly necessary.
That's my understanding anyway, but I'm still just learning about peer-to-peer and Stratus.