FlightRadar24 (and indeed any flight "radar" that consumers have access to) relies on ADS-B data, which is transmitted by the planes themselves. Any sensitive military aircraft would (should) not be transmitting this data, and so would not show up on these services.
1. Mode S: Even if an aircraft isn't broadcasting its own position via ADS-B, if it's using Mode S its position can be multilaterated if you have a cooperating network of receivers (which is what FR24, ADS-B Exchange, etc. have).
2. TIS-B: An aircraft could be using Mode A/C, which generally can't even be multilaterated, or it could have its transponder turned off entirely and ATC can still use primary radar returns to broadcast a synthetic ADS-B-type target for safety, so that other nearby aircraft will see it show up on their traffic displays. The data won't tell you what aircraft it is, but you know where it is. And in fact, it's frequently possible for experienced observers to figure out what kind of aircraft it is and who's flying it based just on its track, speed, and altitude. I frequently see contacts on ADS-B Exchange in the Los Angeles area that are obviously fighters flying without ADS-B or Mode S, showing up as TIS-B.
Military aircraft on sensitive operations can turn off their transponders, and I don't know if China uses TIS-B, but I just wanted to clarify that there is data besides ADS-B that gets used, and that a network of receivers has capabilities that a single receiver doesn't.
https://aireon.com/resources/overview-materials/its-just-ads...
https://www.flightradar24.com/blog/satellite-based-ads-b-dat...
But because it's still relatively rare for aircraft to use PIA, and even more rare for a group of several aircraft to use PIA and take off from the same airport and fly together, they attracted the attention of a few people on twitter who suddenly got very interested in these aircraft and who was flying them. People quickly figured out where the aircraft where flying to, and realized that airport had a public webcam. As the jets landed, people confirmed the tail numbers of these aircraft that were trying to fly under the radar, so to speak.
The thread ends with the identity of the aircraft and their owner having been discovered, some hypotheses about the reason for the flight, figuring out which resort the passengers were staying at, and... the airport having apparently seen what was happening on twitter and sending someone out to put a plastic bag or something over the webcam to block everyone's view.
Kind of a 'closing the barn door after' kind of thing yeah? Might. Would have been better OpSec to leave it running until the next event. Then turn it off prior.
I am a total noob in this area (and in general) but from my cursory understanding of Mode S, you would still have to have an active transponder on the aircraft. Wouldn't a military aircraft that wants to hide it's location not output any radio signals (except narrowband directed signals if those are practical?).
The Chinese government is involved with the big companies there, so they must assume the same is true for the US government and companies...
It's not like the US government doesn't know where these aircraft are already and have an abundance of ways to identify whether they're "of interest" or not
In addition to jjwiseman's comment that Mode S tracking and TIS-B are still widely useful (something you can easily see for yourself in many parts of the world, including around the 305th Bomb Wing in Tampa (near my recent home of St. Pete), which are a joy and marvel to watch), I want to add one more comment:
I don't think it's reasonable to take as granted that any particular party (especially those which are taxpayer-funded ) "should" refrain from transmitting surveillance and telemetry data. Rather, I hope we develop a society in which anybody who is flying a large, fast, dangerous piece of metal through the sky is required to report, via such an open standard, these details for all to see.
edit: for example https://news.ycombinator.com/item?id=29152224
It would be just another government plane, but they went and made it interesting.