The measurement is made by the entire universe as gravitational waves spread out, and interact with, and affect all matter and energy.
REP:The measurement was made only about the existence or non-existence of a Black Hole by the Universe containing it.
No other information is available.In principle the black hole can be Anything!!
Anything can also include events which are beyond Physical Laws....
So in principle Mathematical Physics leaves a space for its own violation....this defeats the Spatial integrity of Mathematical Structures.
Which in my opinion should not be allowed ...
AND therefore my dear friend the Black Holes must communicate.