Attention is a task dependent context. I can drive for hours with no incident but be bored by a meeting in 5 minutes. Very little to link the two, because they're not the same by any metric outside of time. To average that out is oversimplying to absurdity.
Secondly, this myth has been debunked _thoroughly_. I don't have offhand access to the papers but I will add some write ups about it. But the state of attention report report 2018 is a good place to start if you feel like a Google hunt.
Third, most all of the theories around a shrinking attention span are based on unreplicated drivel. And it is drivel. Psychology is a new field still figuring out it's limits and medicine predates science and has a lot of falsehoods-presented-as-fact (this is a huge problem and contributes to the rise of anti vaxxers). Not a lot of evidence so much as opinions.
I find the idea that I comprehend something less than if i had it on paper to be hilarious. I grew up on screens, maybe I'm just accustomed to it, but i have decades of learning mundane and complex subject matter via various tech portals. There is so much evidence that this happens on a large enough scale to dismiss this nonsense here and now