What are the advantages of Android apps being single threaded?
Android apps aren't single-threaded. Although there's one main thread as described in the documentation you quoted, apps can start as many other threads as they need.

The advantage of all GUI interactions being on one thread is that the GUI code can be simpler, faster, and more predictable, because GUI state isn't being changed concurrently from different threads. It also makes thread-safety easier for app developers by providing a message queue, similar to those found in other OSes' GUI frameworks. The app author just needs to make sure all long operations are performed in background threads, sending messages back to the main thread to update the GUI, and that way the GUI remains responsive all the time.