This is such a strange issue. There's 2 possibilities: this is real OR the US gov is punking us, ror more specifically, China and Russia. Why else would you deny UFO's for a century and then start leaking classified footage all of a sudden?
Tom DeLonge, an ex-member of the band Blink 182, is a UFO believer and professional enthusiast trying to educate the public on the "truth" that real UFO's exist. He claims to have had meetings with high level members of the Pentagon who have told him they've had contact with aliens. I believe that he believes everything he says.
It's totally possible that the US gov is punking Tom, unbeknownst to him, to help him spread their propaganda. But really, we don't know anything for sure.
Even weirder: Former Canadian Minister of National Defense (1963-1967) Paul Hellyer claims that the US gov told him they've had contact with aliens.