If it was a foreign military's advanced aircraft, this is them admitting that, yep, your stealth/whatever technology works great, and we are baffled by it.
Why would the pentagon do that? It seems much more reasonable to keep it a mystery and imply that we have an even more advanced tech that they don't know about.
Okay and then to engage the 14 year old in me who used to watch alien shows on TLC: wouldn't this be a great strategy for disclosure about alien life?
1: No aliens do not exist, and you are crazy for it!
2: Well there are some technologies which are just really advanced
3: Well there are just some technology which is really advanced and we don't know what it is.
4: Well there have been a bunch of encounters our pilots have been having with something.
5: Well maybe that something is very unexplainable.
6: We might have some materials from another world, but the only people saying so are crazy!
If I was in charge of disclosing to the world that there were aliens, this is how I'd be doing it. Extremely slow drip, people argue about the details of it (well yeah it's alien tech but they haven't been here for 3000 years, and will probably never come back vs no I think the aliens visit all the time vs no THIS is all the conspiracy there are no aliens etc.) until it's finally just an accepted fact that aliens exist and have visited earth or are actively visiting earth.
Very strange.