They may be lying. However, I doubt China has more advanced aircraft than the US unless they stole it from someone else.
China has nothing. If they did we’d have all been speaking Chinese for centuries ...
Whose are they? I’m going with the civilization that preceded ours and was destroyed in 10,800 BC during the 100 year comet strikes (which ushered in the Younger Dryas Event and killed all the mega fauna).
They took their stuff and left - recently, in the past few thousand years, they have been returning - now they are getting a bit more aggressive.