As I read it, it may not even be actively responding at that time it might just mean it is set to respond, the wording is unclear in the manual.Newly revealed UAP Task Force Chief Scientist Dr Travis Taylor believes the Gimbal incident may show evidence that the craft communicated that it was friendly.
[...] the ID Friend or Foe unit on board the aircraft appeared to have elicited a response from the UAP in the affirmative, ie that it's friendly.
Identification, friend or foe (IFF) is an identification system designed for command and control. It uses a transponder that listens for an interrogation signal and then sends a response that identifies the broadcaster. IFF systems usually use radar frequencies, but other electromagnetic frequencies, radio or infrared, may be used. It enables military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator.
Mode 4 – military only; provides a 3-pulse reply, delay is based on the encrypted challenge.
How does this work?
Usually, a (secondary) ground radar sends out a pulse and the transponder replies with a code that is then displayed on the radar screen. For example, civilian aviation uses mode 3, which is a 4-digit octal number that any pilot can freely set on their transponder; setting it e.g. to 7600 would tell the radar operators your radio is not working. Usually pilots get assigned this number ("squawk") by ATC so the traffic controllers can track them on their screens. (This system has largely been superseded by ADS-B.)
For military applications, this isn't secure enough: once the enemy detects the "passcode", they could simply set their transponder the same and masquerade as friendly aircraft. That's why this mode is a cryptographic puzzle: the radar station sends a random challenge number, then the aircraft transponder calculates a response number using a secret key and sends that back (encoded as delay time), and then the radar station checks that it's correct. Because the enemy doesn't know the secret key, they can't send the correct response; and because the challenge is different every time, they can't just copy someone else's response to another challenge because it wouldn't match.
So what happens with this system is that a radar station sends a signal to the aircraft, and the aircraft responds.
There is absolutely no reason to assume that the mode 4 signal was sent from the UAP; it probably originated from a NAVY radar somewhere in range.
There is no way for a UAP to identify as "friendly" on mode 4 IFF unless they have cracked the crypto. If extraterrestrials can do this, they can contact us some other way.
And anyway, the HUD message means "the transponder is replying to valid mode 4 interrogations", i.e. some radar is interrogating the Navy aircraft, and it is responding. The idea that the Navy aircraft is interrogating the UAP on the ATFLIR display is misguided, because the aircraft is not doing the interrogating. (Presumably you'd need to be flying an AWACS to be able to do that?)
As a "professional" UAP analyst, Taylor (again) shows an embarrassing lack of technical understanding for a "chief scientist".