When I said 'openly intruded' I was refering to surface ships, notably the 1988 incident at black sea. Obviously one does not need advanced ASW sensors to see if that was happenning post-cold war.
During the cold war, if a US ship was strand or somehow runing into trouble near the soviet coast, that's not really a big deal. A small crisis maybe but those days we saw worse crisis every year. It would be a different story today.
Technically, within the EEZ of China the average depth is 100m or so and traffic/fishery is extremely busy, the best technology on the earth cannot guarantee the subs could operate undetected or problem-free forever, especially if the operation is kept in high tempo round the clock. If you sail along the coast during fishery seasons, it wouldn't be difficult to realize that evading the nets and boats alone would be a big headache and severely hinders the maneuverability of any submarines. Often periscope depth is the furthest you can dive, and you probably won’t come across a safe diving depth for hours. Not the best battlefield for submarines of either sides. It's not about stealth or sensor suit, it's about physics.
Occasional intrusions for intel collect etc. are perfectly possible, especially under the cover of bad weather. Routine operation too close to Chinese shore is an unnessicary aggressiveness and the return is poor compared to both operational and political risk.