Yeah I think we can assume that like a third or a fourth of humanity is asleep at any given time. The likelihood of everyone blinking at the same moment is still minuscule even after this of course.
You could certainly have “everyone who was awake blinked at the same time and that let the Eldritch Menace in” as a story’s hook but it doesn’t sound as good as “everyone blinked at the same time”.
