It means exactly that.
The FAA ATC system identifies ALL aircraft with a call sign, in communications. Usually, begins with ‘N’ for domestic aircraft. There are others, SAMxxx and AFx (AF1/AF2), and so on. bidEn as POTUS has flown places, but if you follow the aircraft on various ATC trackers, you will NEVER see him on ‘AF1’. It is always “N/A” or “empty” or perhaps some other variations.
This is also made more the curious by noting that Kamal has ALWAYS flown in AF2.
But, she is not Constitutionally in line for Commander-in-Chief.
I am always reminded of Al Haig’s comment when Reagan was shot, “We are now in control.”
Where was Al Haig? [AF1]
What “equipment” would he have been accompanying? [Football]
Constitutionally and according to Law, he was correct.
If POTUS is unable to execute the office, the Joint Chiefs take on C-in-C duties.