It is not send back confirmation immediately, if the Render process is busy on other task, the ACK message will send later. 
Our method definitely differs from just waiting 4 secs after amount of runtimes==1 and closing the main runtime, one reason is check 'REAL IDLE' status as discussed above. The other reason is your proposed method just checks whether browser process(aka runtime process) is ready to suspend the main document, it doesn't consider whether render process is busy or not. The IPC between browser and render process will make sure both process are available to close the main document.