A non-pure OO language in this context is a language were not everything is objects. Java for example have "primitive" types, which behave differently than objects. For example, Java's primitive types are passed by value, while Java passes references when it passes it's objects.
In contrast, Python treats everything as objects, and although it has built-in types, it has no primitive types, everything is objects.
I think you misunderstand the reference to classes. I don't think they say that a pure OO language does not have classes.