US News

Could all be smoke blowing bullshit to try and scare the Chinese. Make them think America has these super-advanced alien-derived beyond beyond next generation super wunder-craft that they'll have no choice but to unleash in a direct conflict. The goal being to give more weight to US credibility to try and dissuade them from challenging the US in Taiwan or elsewhere when they act.

However it really doesn't matter, unless these craft have energy barrier force fields around them or can move absolutely ridiculously, impossibly fast (doubt, they'd have applied it to interceptor tech or a next-gen missile program by now but they still lag behind Russia/China) they can still be hit and taken out by hyper-sonic missiles even if they could give them a theoretical advantage in dog-fighting with jets.

Come to think of it that seems as good a reason as any as the US is increasingly lacking credibility because of Ukraine, inventing this myth that we have these super secret alien-derived wunder-craft which we've been hiding and avoiding using would do wonders for the psyche of the western militarist which is going to get quite a rattling soon when Ukraine loses. They can just claim we didn't dare unleash them there because we didn't want Russia to have a chance to study them and are holding them in reserve for any kind of direct conflict.