From a reader…
I think the layout of the place makes a difference.
For example, if the overflow space is continuous with the church space, and there are screens, I think that is attendance. Even if the people are outside because of the overflow, surely they are present even though they are inside. They are morally present in the church.
However, if the overflow space is not continuous with the church space, if the hall or place where overflow is expected to be is in a different part of the building or in another building, I think those people are not attending Mass. They are attending the transmission of a Mass. They are not morally present in the place where Mass is being said.