I reckon it's misdirection. "We'll say it's a UFO, but really it's a hypersonic plane in development." Just like the SR71 had a cover story during its development phase. "The wreckage was recovered in two days, and persons at the scene were identified and requested to sign secrecy agreements. A cover story for the press described the accident as occurring to a F-105, and it is still listed in this way on official rec…
That makes it look like you're trying to mislead/lie/coverup.
It's a good strategy to advertise new capabilities to adversaries and/or let them know you know about theirs or just to make them think rare atmospheric/sensor phenomena are scary new tech.