As someone who arrogantly assumes that I'd be in the 5% that had a "detailed, literal understanding of the first paragraphs of Bleak House" since I achieved a 36 on my Reading ACT many years ago (and yes, I feel like a loser even writing that), I'll give a tiny defense of at least some of these participants and their results.
The study says that:
Students read each sentence out loud and then interpreted the meaning in their own words—a process Ericsson and Simon (220) called the “think-aloud” or “talk-aloud” method.
I feel like that, in combination with the potential stress of the situation, might lead to really stupid sounding answers, like some of those quoted in the article. I personally tested myself using the "mud" example, and while I think I gave a passable initial answer, a verbal answer that accurately matches and translates the original text on a sentence-by-sentence basis is fairly difficult to construct verbally for me, at least within the first pass. That job of translating the text into modern English is difficult, and is a synthesis of information that requires far more cognitive reasoning than just understanding the text. Give me a pen and paper, and I think I could do a far, far better job, and I would assume the same of a lot of the participants.
Additionally, many of the words and phrases in those samples are very archaic. Participants were allowed to search up definitions, which would definitely help to clarify those archaic terms, but again, I'll note that this seems like a stressful test, and participants may feel like they're being negatively judged for even looking up terms like that. One of the examples highlighted by the article could even be interpreted as showing exactly that:
And I don’t know exactly what “Lord Chancellor” is—some a person of authority, so that’s probably what I would go with. “Sitting in Lincoln’s Inn Hall,” which would be like a maybe like a hotel or something so [Ten-second pause. The student is clicking on her phone and breathing heavily.] O.K., so “Michaelmas Term is the first academic term of the year,” so, Lincoln’s Inn Hall is probably not a hotel [Laughs].
[Sixteen seconds of breathing, chair creaking. Then she whispers, I’m just gonna skip that.]
The article uncharitably attributes her behavior to "the cognitive load of reading these archaic terms and complex sentences," but I don't know, that just seems like plain test-related stress to me.
I mean, the study itself seems to disagree with that sentiment: