You're correct about the study. What they actually found was that a certain fraction of UFO reports (I.e., what loldrup calls UO) had reported descriptions that didn't match any known class of object. So yes, it's more like P(UAP|UO) in loldrup's notation; and yes, it's not "thing known not to be a known class of object" but "thing whose reported description we didn't find a good match for" which is of course consistent not only with what loldrup calls the UAP hypothesis but also with inaccurate reporting and with known classes of object having currently-unknown behaviour.

[EDITED to fix formatting.]