So Captain Fravor is s liar? He claims to of seen the tic-tac and chased it. So if it’s a secret jet, he’s a liar, or we developed anti-gravity tech in secrecy somehow.
I'm not saying that happened, just that it is a possibility. Personally I think it's aliens.
There's another possibility, too; he's human, and humans frequently make mistakes. Eyewitness reports are incredibly unreliable.
And, again, the military lying to uncleared civilians is hardly without precedent.