Hola Stuart

When we talk about people in general and there is no noun/antecedent we use the masculine form in the plural; this doesn't only happen in comparative/superlative sentences like this one in the lesson, but also in other sentences. I imagine, although not entirely sure, that we use the masculine form as in referring to "los hombres más..." which refers to a general men and women but used the masculine as default. 

Los más ricos tienen que pagar más. (= la gente más rica) 

Rich people (the rich ones) have to pay more.

Los pobres son muy desafortunados. (= poor people, la gente pobre)

Poor people (the poor ones) are very unfortunate.

Debemos respetar a los mayores. (= elderly people)

We must respect old people.

I hope this clarified it.

Saludos