I guess it would be "intelligent agencies acting outside the bounds of what we commonly accept to be the limits of the natural world” (whatever that means)...
Is it testable? I guess quite possibly. If UFOs created by "intelligent agencies acting outside the bounds of what we commonly accept to be the limits of the natural world” (whatever that means) are real and are buzzing (or humming, or thundering or just being silent) while flying around as described by UFOlogy lore, then at least one of the pieces of reliable evidence exposed below should be available.
1. Clear sharp imagery with the propper provenance of something just like the shape-shifting UFO allegedly seen over Iran or the one allegedly seen by Father Gill or the one allegedly seen by Zamora or the one allegedly seen by the Hills or the one allegedly seen by Villas-Boas...
2. Reliable records of radar contacts of UFOs performing the alleged maneauvers which are physically impossible or beyond our technical reach...
3. At least a piece of hardware beyond our technology, preferentially with isotopic composition indicating its not from this world...
4. A specimen of an "intelligent agent from outside the bounds of what we commonly accept to be the limits of the natural world" (whatever that means)...
Hey, UFOlogists, got some of the above? Note that rumours about Area 51, Roswell et al., rumours about alleged physical effects, eyewitnesses reports as well as images which are blurry and/or suspected of being hoaxes images do not meet the standards.
The answer is: No.
The outcome? Hypothesis fails.