It's a good recommendation. Even if someone disagrees with it, it's the ideal primer in understanding what the arguments and ideas behind classical/marginalist/Hayekian economics actually are (rather than just hearing about them secondhand.)

How can it be right if it's middle-school level?

How can a textbook saying "the planets and asteroids orbit around the sun" be right if it's middle-school level? Or one saying "life is generally divided into five kingdoms" be right if it's middle-school level? In each case, there's some added detail and complications ("actually they orbit the 'barycentre' of both bodies", "viruses are hard to categorize, and some lifeforms are obligate symbiotes") but the essential facts and basic logic of the theory is there.

My point: it's written in middle-school language, aka plain language.

It's adult-level economics (although I'm not sure what that even means).