In case anyone did't get the "disputed" part, Vietnam occupies most of South China Sea islands especially in the 70s and 80s around Sino-Vietnam war. China (actually Republic of China) claimed them earliest but didn't occupy.
If you google "nine-dash line", which violates UNCLOS, you'll understand why the ASEAN countries are upset and rightly so. China has visibly flexed their muscle in the region in recent years with building military bases, adding land to underwater islands so they can claim sea territory around them, and attacking fishing boats in disputed region. Some of this happen to dispute with Japan as well. With the economy not going well, I'm afraid this may be the Communist Party's strategy to diffuse tensions at home in order to hold on to power.
It's disputed because 3 countries claim ownership of the the island as it's within their "territorial" (with the exception of Taiwan which claims it because it's still pretends to represent "real China") waters.
China pretty much made claim to all of the waters it's claim extends as far as the shore lines of Malaysia.
China also extends it's claim over waters outside of the immediate territorial waters by building artificial islands so far only within the 200 n/M of their exclusive economic zone which it claims extends it's coastal waters even further (this isn't exactly the case for this specific island (it's land mass was artificially extended, and a large harbor was built) but an important background point for the entire dispute).
So far countries have refrained from militarizing the islands too much sure they might post a couple of sailors here and there but this is a long rang air defense system capable of shooting down aircraft as far as 400km away, this is basically an S-300/400 [https://en.wikipedia.org/wiki/S-400_(missile)] "copy" with a much more advanced radar and better interceptors by all counts.
S-400 long range capability is mostly theoretical, as there's no stock of the 40N6 missile capable of it. Especially doubtful in case of China, given the secondary nature of their missile technology.
China builds their own interceptors so the availability of Russian stockpiles isn't relevant, they've both deployed and conducted real world tests on multiple interceptors beyond the initial 200 KM range that the system went into service with including having improved interceptors that can counter ballistic missiles and satellites[https://en.wikipedia.org/wiki/2007_Chinese_anti-satellite_mi...].
By all accounts it was a modified HQ-9 interceptor which was piggybacked on a short/medium range ballistic missile (mostly because of the high inclination of the test satellite, which might mean it's quite likely to be potentially capable of reaching LEO VISINT satellites which pass over the interceptor on it's own).
Any space faring nation has technology to intercept LEO satellites. That doesn't mean a tricked out ICBM has any use in intercepting manuvering fighter jet.
The original HQ-9 interceptor is stated to have 200km slant range, just as the Russian missile it was faithfully copied from.