If I understood Tyson correctly, his "argumentation from ignorance" means something like "Since I know of no scientific explanation, it must be ..." (enter your favourite explanation, scientific or not) - a clear fallacy. (In contrast, "I guess ..." is perfectly permissible.)

The notion "Unidentified Flying Object" consists of three words; the first one becomingly admitting some ignorance, whereas the other two do make assumptions: a) the sighting is caused by light coming directly from an object, and b) the object is "flying", i.e. solid and propelled by a force beyond mere gravity. In other words, most "UFO" sightings are not sightings of FOs at all.

"People are so stupid; they always believe they are right, and fail to realize that in fact I am the one who is right!"