I'm curious also.
Is this implying that JMSDF is effective at detecting PLAN Submarines...
The JMSDF has been the protegee of the USN since the beginning of the Cold War when it comes to the ASW domain. The geography of the Japanese major islands means that they naturally become perfect chokepoints to guard against Soviet subs from freely entering the Pacific in case of war. Japanese warships and aircraft procurement too has historically been tailored with significant focus on the ASW domain, in order to further piling on such advantages.
Just take a look at the number of P-3s and P-1s they operate, for instance. Not even China has that many KQ-200s, in comparison (which hopefully can change in the coming years as China looks further into the "true blue" Pacific).
The JMSDF earned the nickname "The ASW squadron of the US 7th Fleet" for this reason. Needless to say, the JMSDF's expertise and sophistication in this field is second only to the likes of the USN.
And given the considerable disparity in terms of size and capabilities of the PLA and JSDF in the naval domain, especially in the underwater and counter-underwater domain for a long time until rather recently - It isn't really surprising to hear things like this either.
Remember, China today still operates a nuclear submarine fleet which isn't just small in size relative to the overall size of the PLAN, but also primarily consists of subs that are at least one or two levels below their American and Russian counterparts, if not more.
The much-anticipated effects from the massive "black fish breeding" activities at Huludao won't bear fruit until the second half of this decade, so there's that.
...such that PLAN subs will be quickly neutralized during open conflict?
If so, that would seem to be very worrying to the PLAN....
As mentioned by others above, the PLA today has much greater means to mitigate the threats posed by JMSDF assets against PLAN subs than ever before. If the JSDF cannot station their assets in and close to China's vicinity due to the acute dangers posed by various other assets of the PLA, their ASW capability can be seriously degraded - Hence giving more breathing spaces for PLAN subs to operate around.
Furthermore, I do think we need be thankful that we aren't left with the PLA from the 1990s, 2000s or even in the early-2010s - Those times were indeed bad, if not desperate (or even outright hopeless).