There is no value is advertising to your opponent that you are capable of sneaking into the middle of your task group undetected. That would be like the Japanese moving their fleet to within 200 miles of Pearl Harbor and then sending a radio message announcing that they're here. If the Chinese are certain that someday they'll be in a shooting war with us over Taiwan then it makes more sense for them to practice taking out the carriers and remaining quiet about their success or failures.
You're thinking like a westerner, and with all due respect, your Pearl Harbor analogy is off. The IJN was built on the theories of Alfred Thayer Mahan and the ideals of Nelson. It was a main battlefleet modeled on the Royal Navy & designed to confront its opponents.
The Chinese are operating on eastern principles. It's all about misdirection. Sun Tzu: appear strong when you are weak & vise versa.
As to the incident itself. I'm not really all that impressed. It was restricted waters. A sub of that type popping up is akin to running over a mine. Plus, we won't really know if a Los Angeles had it all along, because WE don't advertize our capabilities.