The main value of these sorts of detections is towards probing the early ages of the universe and how quickly certain things formed, in which context, the idea of a larger black hole beyond the edge of the observable universe doesn't make much sense, since the edge of the observable universe is t=0 for the big bang (to be precise, the edge observable via photons is actually t=~370k years, the point where the universe cooled enough to be optically transparent, but theoretically we could look to t=~1 second if we could devise a way to detect relic neutrinos from the big bang).

Fact is, far enough in the future inflation will be such that our view is vastly smaller than it is now. Their universe will be smaller because there will be no way for them to access what inflation has put beyond light and causality.

If it's outside the observable universe then by definition it has no observable effect. Until technology improves to expand the observable universe to a degree that we can observe it or it does something that allows us to observe it (which adds it to the observable universe), then it doesn't really matter to us.

In other words: If it's not part of the observable universe, it doesn't matter to us because we can't possibly know anything about it. However as soon as we discover something about it, it becomes part of the observable universe and now we can care about it because we know of it.