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.
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.