Maybe better say ‘zoo’ vs ‘forest’, or ‘very well protected area’ vs ‘partly protected area’.
If there is only a few habited planets inside grabby alien sphere, they will be very valuable and very well protected so no UFOs will be observed.
If there are millions of them, they are less valuable and thus less protected and therefore can be used for some practical activity, like turism, hunting or mining unobtanium. Obviously, if UFOs are aliens, local alien authorities let them be visible sometimes, so local aliens laws are not very strict.
Observation selection effects like SIA favors the hypothesis that there millions habitable planets inside any grabby aliens.
I found it rather difficult to download it from any of the listed sources, except amazon kindly web where formatting is not good for reading. Can you just upload a pdf somewhere?