Including personal information on your work calendar without using privacy controls is pretty bad practice, though obviously some people occasionally did it. There is no indication that this information was a target of the fired employees. There is also no indication that these personal events were included in screenshots “outside the company”. And that phrase encompasses a wide range of possibilities, I suspect that Google made it so general because the specifics would have sounded more benign.
The statement in general is designed to cast the four in the worst possible light.