Probably because we are more experienced than China at this sort of thing.
The 'Big problem' here is not for US, but for China. The US is getting ready to field a '6th gen' fighter while China is STILL working to field her first 'stealth' fighter. So who is the more believable here ? China can say whatever she want. WE DO NOT CARE. What I said was just MY OPINION. There is no 'Big problem'. You are free to believe China whatever she said about 'stealth'.
Whose 'stealth' fighters have actually been in combat ? US. Not China. And please do not bore US with the single F-117 loss over Yugoslavia. No one who has actual working radar experience -- that includes me -- take that event seriously. I have post plenty of technically oriented rebuttals on what happened and so far, no one have refuted me. No one -- not even the Chinese members of this forum.
So not only are we moving towards the next generation of 'stealth', our current version have been in hostile airspace. China ? Pfffttt...!!!