Is this actually true? My current guess is that even though for a given level of training, smarter people can get through harder texts, they will learn more if they go through easier texts first.

I think you're right here (enough so that you beat me to my reply nearly verbatim.)

Furthermore, a hard to use text may be significantly less hard to use in the classroom where you have peers, teachers, and other forms of guidance to help digest the material. Recommendations for specialists working at home or outside a classroom might not be the same as the recommendations you would give to someone taking a particular class at Berkeley or some other environment where those resources are available.

A flat out bad textbook might seem really good when it is something else such as the teacher, the method, or the support that makes the book work.