That is not much of an "explanation" as far as I'm concerned. The truth is that the Army "air service" went by a lot of names over the years, but never, until after WWII, was it know as the "United States Air Force." Check out the following:
During WWII, the service was reconfigured and named the "United States Army Air Forces" (note well the plural "Forces"). There simply was no USAF prior to 1947, and to refer to the 50th anniversary of the founding of the Aeronautical Division of the Army Signal Corp as an anniversary of the "USAF" is simply awkward and incorrect. No one talked that way, not in 1947, and not in 1957.
I cannot help but believe that the stamp was designed by someone not intimately familiar with the history of Army aviation, and by the time anyone realized it it was too late to scrap the stamp. Not like that hasn't happened at other times! So the PB came up with this awkward "to commemorate the 50th anniversary of the United States Air Force as part of the National Defense System."
Hardly anybody any longer used the term "national defense system" in 1957, but it was the best they could come up with.
Explanations given after the fact strike me as ad hoc and unconvincing. Sort of like "well, we endured 26,000 casualties on Iwo Jima so B-29's would have an emergency landing strip." The only thing that could change my mind would be evidence from the time period when the stamp was designed that explains why the term "United States Air Force" was used.