The app can do whatever else it wants to do that's completely unrelated to the data request. The part of the app that requested the data is stalled until the data arrives. If the data request is the only useful thing going on (e.g., a user clicked "search for Joe's phone number") then there's nothing else productive for the app to do and async I/O is irrelevant.
Yes. That last bit is very rare, especially in server applications where you have multiple clients. In any case you might want to be a little more careful before calling something that you didn't understand nonsense.
