This is totally right, but I think the majority of people understand this, and use "screen time" as shorthand for completely passive and solitary use of screens

This is totally right, but I think the majority of people understand this, and use "screen time" as shorthand for completely passive and solitary use of screens. I know many people really do say it is the screen itself that's the problem, but I don't think that's the most common colloquial use of the term.