Here's my take on it: the ATC chatter you pointed to shows pretty clearly that there were still plenty of aircraft in the sky at the point where UA 93 went down. There's was also a lot of discussion on the radio between the controller(s) and those aircraft. United has a service where you can listen to the current ATC frequency that the plane you're on is listening to on "Channel 9", and I've listened many times; there is often this kind of collaborative conversation going on about things on the ground like fires, weather, other planes, etc. -- sometimes even just landmarks ("What's that big bridge down there?").
Two items:
- If there were fighters in the air nearby, they would have HAD to be in contact with the controller for that sector. The controller, even up to the time that the smoke on the ground is seen, does not appear to have any idea that UA 93 had been hijacked. He continues to try to raise UA 93 and doesn't hide the fact that he (and others) heard someone say "There's a bomb on board, we're going back to the airport" ... if he had realized that it was a hijacker mistakenly transmitting on ATC rather than on the PA, he might not have let the hijacker(s) know that's what he was doing.
He also does not communicate with anything military, nor give traffic advisories to the other aircraft about anything military in the area. Military aircraft often (but not always) use a different frequency to talk to ATC (for technical reasons, not for anything 'secretive'), but ATC uses "simulcasting" to talk back to them on both the military frequency and the civilian one, because it's useful to the other pilots around to know that there's military aircraft in the area.
- If there were fighters in the area, and on frequency, there's a dozen or so pilots out there who are keeping this quiet (plus a whole room at Cleveland Center). That seems very unlikely to me.
The ATC MP3 was heavily edited and time-compressed; if anyone knows of a source of a longer one, I'd be interested to hear it.
/jordan