As used here "UFO-crazy" wasn't a supposition, it was a constraint.
"UFO-crazy uncles" are known to exist. This is not an extraordinary claim. The existence of such uncles provides no evidence for or against extraterrestrial visitors or other aerial phenomena.
In context seemed more like a smear for any who don't dismiss as unremarkable. But I'm glad you took it as the narrow case, tho - do they really "exist", or might they have just been right all along? Lol
The existence of UFO crazy uncle's does from a probability sense have some bearing on whether we have any true extraterrestrial evidence.