Does seem like a strange omission? Especially with Y-9's endurance being subpar compared to airliner based AEW&C's.
To be fair to the PLAAF, pretty much most of their warplanes (other than fighters) do not have mid-air refueling capabilities. The first successful mid-air refueling for China was done between a HY-6 and a J-8II in 1991 - More than 3 decades after the US had its first KC-135 entering service with the USAF.
In the meantime, throughout much of its existence since its founding in 1949, the PLAAF was overwhelmingly focused on national territorial defense, which does not necessitate warplanes with mid-air refueling capabilities, as their home bases are often just an hour or two away. It wasn't until the beginning of this century when the PLAAF begins shifting to a dual offensive-defensive in both tactical and strategic domains, which is where long-range/expeditionary missions start to become more crucial and received more attention than before.
Speaking of the KJ-500 - From what I can gather, the endurance stats listed is around 8 hours. This would require 3 airframes in order to enable round-the-clock on-station at one specific area. However the
actual number required in actual wartime scenarios will certainly be higher.
To enable sufficient 24/7 airspace monitor and control in a full-scale war, China absolutely needs
more AEW&C aircrafts than it has now - In fact,
way more than what the US&LC are currently planning to obtain.
