I think this analysis is pretty accurate. Since there's no indication of online status in Wave, you can see a message come in (in real time), but you are under no obligation to respond right then and there. You certainly can, but you can also treat it like an email and save it for later.
However, I must admit that watching someone type a message which you do not immediately respond to makes you feel like a stalker.
The title is a bit misleading. The concept of Passive-aggressive behavior is something different, and you can do it through email, phone...
Synchronous/asynchronous would be more accurate.
So far Google Wave is like Google Docs with a better diff if not using extension. It will replace Google Docs before it can even replace Gmail.