That theory is as valid as anything else. Given enough time self replicating probes could cover everything. But it might be a mistake to believe that alien AI’s would do this or that just because we have a hard time imagining them doing something other than what we ourselves or our own hypothetical AI’s would do. It could be they would be satisfied to never explore anything.
Also, there could be tiers of power between AI’s, it could be the oldest AI become isolationist and kill all the new ones once they really start expanding.
FReegards
What if Earth was, hypothetically, within the domain of just such an ancient isolationist AI.
And what if Earth was, hypothetically, something like a hobby for said AI.
How would anyone know?