It's supposed to be prescriptive because its a literature education list.

Where we may agree is that Mensa isn't in the business of measuring education, but instead of horsepower.

And the reading way toward maximizing a developing child's base ability to think is to read as much as possible, which will necessarily be what piques their fancy much of the time.

But literature education and recreational reading shouldn't mean one doesn't engage in the other. Every high performing child does both, at least to meet the the standards with which I'm familiar.

Still I'm unsure as to why Mensa feels the need to cobble together a basic prep school reading list, if wildly uncurated.

To your point, something more in line with their rasion d'etre might be to provide short best of lists for recreational, slightly advanced reading in age appropriate genres.