Yes, in many languages, there´s not such a thing as a double negation. It just doesn´t make sense. However in Spanish it´s used here and there and you stumbled upon one of those.
If you take the sentence and translate it word by word to English it would mean that everyone else is smarter than she is but the actual meaning is just the opposite.
The same double negation "paradox" applies to other sentences like:
- No tengo nada (I have nothing)
which could be understood by an English as a convoluted way of saying that you actually have something.
It´s also worth noting that the simple negation equivalent would make no sense to a native speaker. Saying something like:
- No es más lista que alguien
- No tengo algo
make little sense and may lead to one or two confused faces.