Anything can be "claimed." Some countries have arms to sell and hype is build to justify sales.
Fact of the matter is that Americans know what they are doing.
Historically, only Yugoslavia managed to shoot down one F-117A Nighthawk in war but the overarching intercept rate was negligible. F-117A logged over 700 sorties in Yugoslavia and B-2A logged around 50. Now bear in mind that Yugoslavia had taken notes from the Persian Gulf War (1991) and created IMAD defensive arrangements with radar systems that could detect stealthy aircraft in theory.
Syria have sourced A2/AD components from both Russia and China including an
S-300 class battery from Russia and YJ-27 class radar system(s) from China, but they have no counter to Israeli F-35i.
Refer to this incident. Israeli aviation experience is considerable, and they have used even F-16 variants to penetrate Syrian A2/AD arrangements in daring raids.
Interestingly -
Syrians acknowledged that Chinese radar systems are much better than Russian in their arsenal. Good marketing for Chinese equipment in this case.
Chinese have developed more advanced radar systems by now that are optimized to detect stealthy aircraft in theory but nothing is proven in operational terms. There is utter lack of operational data to inform development of robust counter-stealth capability. There are a lot of variables to account for in this game. There is the whole Electronic Warfare (EW) spectrum to this game.
NGAD development is informed by decades-worth of data and research and with "emerging threats" in kind. This will be a repeat of F-22A Raptor saga - a jet fighter which is feared to this day.