Does it mean that we are, potentially, on one of two poles(?) of the observable universe, if we're observing most galaxies around us rotating a certain way?
It would also imply that our whole universe is rotating - the only reason this happens on Earth is because of our planets rotation and the Coriolis effect.
[^]: Ignoring ejections. But black holes also don’t “eject” mass. Or maybe they do? Hawking Radiation is weird.
Similarly, the dynamical spacetime around a black hole not near any other black hole can couple with quantum fields -- even fields in a no-particle "vacuum" state as measured by an observer, for example one in orbit around the black hole -- with the result that Hawking radiation is produced.
Both gravitational radiation and Hawking radiation carry away energy (in the sense of ability to do work, per the "sticky bead" argument) from the environment immediately around a black hole. This in turn means that the horizon radius will be less than it could be.
So as a Hawking-radiating isolated black hole will tend to shrink (if it's not fed by hotter cosmic microwave background radiation, for example), the mass of a post-merger binary black hole will be less than the sum of the unmerged binary.
Just because things can't cross from the inside of a black hole horizon to the outside doesn't mean the horizon is always the same -- the horizon can grow and shrink dynamically when interacting with other self-gravitating bodies, with matter like dust or starlight, or with "the quantum vacuum".