> I have younger relatives who can't sustain their attention to read a book to save their life but still earn excellent grades

Can they sustain their attention on dense and technical things at all, or when there is no grade involved?

Pointing to school grades is not really a good measure of "can these people actually digest and understand complex and longform information and narratives?" The relevance of that requirement should be obvious: at many points in your life you will need to manage boredom and your attention, to understand boredom and focusing for a longtime as a part of life and learning.

When I was a TA in uni 5 years ago, many students found reading anything longer than 8 pages to be interminable or downright impossible, which I found rather pathetic. They would give up. These were all kids who got excellent grades. They couldn't accept or manage their boredom at all, even if it was just a part of learning to do things. They constantly wanted summaries, which to my mind is worse --- they wanted someone to tell them what and how to think about something without engaging with that thing themselves. We all have to do that sometimes, of course; but, we should not expect that to be the default. What they lacked more than anything was intellectual curiosity.

Remember when films used to be a tight 90 minutes of snappy editing. Now everything is getting close to 3 hours, it's not because the stories are better or more complex it's people not being ruthless in their editing.

I remember struggling to read dense texts at university. As I've aged and read more, I'm pretty comfortable in the belief that most of the stuff i had to read wasn't that good and was just a boring slog purely because the author liked writing words.

Writers like writing, Readers like reading, and sometimes what they both would benefit from is a ruthless editor to focus their effort.