Military radar systems have modes and methods for detecting or removing jamming:
https://en.wikipedia.org/wiki/Radar_jamming_and_deception#Co...
"The most important method to counter radar jammers is operator training. Any system can be fooled with a jamming signal but a properly trained operator pays attention to the raw video signal and can detect abnormal patterns on the radar screen."
So they do have the ability to look at the raw signal, and other methods as well to try to tell if something is real or not. Also there may have been multiple types of radar systems (different aircraft, ground vs air, different software revisions, different approach angles); it would be very difficult to fool all of them at the same time. For example if there were three aircraft, each at a different approach angle, the false signal would have to adapt to that approach angle individually for each aircraft's radar (or the supposed software glitch would have produce coherent results that are adapted to each aircraft's approach vector so that they all "see" the same thing in the same place). The same is true for false signals from natural sources. Which is not very likely unless you're dealing with a sophisticated human attack on the radar. To fake signals for multiple sensors on multiple approaching aircraft at the same time would require an even more sophisticated attack. Who knows if that is possible, but it does not seem very likely, especially in a peacetime operation during which there would be little reason to reveal such a capability.