I think this is both a response to the previous incident where the Japanese scrambled 8 F15s and support aircraft to intercept a single unarmed Chinese Y7 transport over the islands, and also the PLAAF calling Japan's bluff when Japanese officials suggested Japanese fighters might try firing warning shots at Chinese planes.
I think the previous incident with the Japanese scrambling 8 F15s was very interesting in a number of ways.
Firstly, I don't think anyone would dispute that 8 F15s against a single unarmed transport is overkill of ludicrous proportions, and that overreaction is telling and indicative of the deep insecurity the Japanese are feeling, and for good reason.
The bulk of the Japanese F15J fleet are over 30 years old now, and only 30 have been upgraded with a modern radar and have ARHAAM capability in the form of AAM4s.
This is what the original F15Js' cockpits look like.
And the upgraded F15Js are not that much better off tbh.
I would rate the un-upgraded F15Js as equivalent to the Su27SKs the PLAAF got in the early 90s, with the upgraded F15Js equivalent to later block J11As but short of J11B standard. We all know how the Su27SKs and J11As faired against J10As in exercises, and I would not expect the Japanese F15Js to do much better.
On a one-on-one basis, even the upgraded F15Js would be hard pressed to hold their own against J10As, and the Japanese only have 30 of those. The rest of the F15J fleet will have little chance if faced off against a J10 in BVR or WVR.
It is also interesting that the PLAAF sent in J10s rather than J11s. The Japanese sent in F15Js rather than F2s because the Diaoyu Islands are actually quite some way from the nearest Japanese air base, and they prefer the extra legs of the F15J even though the F2s have more modern radars and avionics (but then, the F2 was designed as a striker, so it may not be as good as the F15Js in air combat even with it's nearer electronics).
The PLAAF's choice to send in J10s might mean one of two things, or both. Firstly, by using the shorter legged J10As, the PLAAF is subtly reminding the Japanese that it's entire modern fighter fleet is in play over the Diaoyu Islands, whereas the Japanese F2s might struggle to reach the islands without extensive tanker support.
The J10As are still the PLAAF's best air combat fighters, and I think the PLAAF wanted to be safe rather than sorry in case the Japanese really were stupid enough to follow through with their silly threats of firing warning shots at PLAAF fighters. If the Japanese were moronic enough to do that, push could very quickly and easily turn into shove, in which case the PLAAF wanted the best chance of winning the ensuring scrap.
Viewed in this context, and when you consider how many modern J10As and J11A/Bs the PLAAF and PLANAF have, one can see why the Japanese were so skittish over the Y7 and over-reacted so spectacularly. The 8 F15Js were not meant for the Y7, but rather Japan sent so many up because they knew the Y7 was sent in to gauge their response and they wanted to send as strong a message as they can to try and scare the Chinese off from doing exactly what they have done recently.
But it did not work. The PLAAF deployed and they deployed in force, with as many as 10 or more aircraft involved. Funnily enough, the Japanese did not send up as many F15Js this time as they did against the lone Y7, nor, thankfully, were they stupid enough to think about firing warning shots, so it all came down to very little in the end. But no worry, no risk of their cheerleaders in the western press contrasting their bullish swagger when faced against a helpless Y7 compared to when they were confronted by PLAAF fighters, so it's all good.