When someone can enter someone's territorial waters ( with a coastline long few thousamd kilometers ) undetected that's not some special thing. But enter, do something important and run away, that's something completely else.
Also, I'm pretty sure that the Chinese submarines can also enter Australian territorial waters undetected.
If you actually read the book, an Australian submarine was almost discovered inside busy Shanghai harbour, because they were dragging a fishing boat under the water to its doom.
If that had happened, the Chinese government could have legitimately and publicly executed the crew for murder...