> They are most likely talking about radar signature physics. Stealth to lower frequency radar requires physically implausible materials.

But then the question moves to: what are the weaknesses of those lower frequency radars, and can those be exploited to restore the advantage of the stealth features? The admittedly not-great Wikipedia page about low frequency radar seems to indicate it has a lot of disadvantages like being difficult to transport, prone to noise, and not being precise enough for targeting.

Advances in missile technology, specifically Track Via Missile, means that they don't need to be precise enough for targeting anymore, they can be off by 100-200m+, but as long as the velocity measurement is good, the missile can fly in the neighborhood, acquire the target, and cooperate with the radar to confirm that they are locked into the same target and maintain a perfect track. They are definitely difficult to transport compared to others, but they are now small enough to be integrated into a truck, and communication is good enough they can segmented into more modules that can be on different vehicles and still work, greatly increasing transportability and survivability (the antenna is cheap, so it can be almost sacrificial as long as the processing is done elsewhere). Susceptibility to noise is a much lesser concern now due to the generalization of AESA technology.

We can now also do sensor fusion, so a low frequency radar can communicate with other radars and even higher frequency radars that wouldn't be able to detect the target in search mode but could obtain some data in track mode to obtain almost perfect accuracy, make it difficult to evade, provide redundancy, defeat jamming, and discriminate against decoys and chaff, as well as provide bistatic capability and help in noncooperative identification. Also crucial is to defeat stealthy antiradar weapons an F-35 might fire in self protection by tracking them from a direction they might not be stealthy in and provide warning or even targeting information to shoot down.

Integrated air defences are very scary, and it's actually incredibly concerning for the USAF that they never fought against a real IADS. Russia is doing so right now in Ukraine and it's clearly not easy even against 40+ year old systems not even being used as designed, and for the same reason no Ukrainian fixed wing has been able to launch any incursion into Russian territory, only helicopters flying way below the radar horizon.