That isn't an estimate of P(UAP). The following two propositions are very different:

• "There are technologically advanced civilizations elsewhere in the universe."
• "Sometimes things happen visibly in our skies that aren't adequately explained by anything uncontroversially known to be real."

The first of these can be true without the second -- maybe there are aliens but they never come here. (That was rather the point of the Fermi paradox.)

The second can be true without the first -- maybe people have actually seen angels, or time travellers from earth's future, or spontaneous rips in the fabric of spacetime.