Jews did consider Arabs to be Ishmaelites in pre-Islamic times. While I don't know of any specific texts saying "Arabs=Ishmaelites" that are pre-Islamic, there are references in early Rabbinic texts to Ishmaelites as a non-Jewish ethnicity who are foreign but are known as a group to Jews.

It's important to note that Jewish associations of ethnicities with the descendants of biblical characters were probably not thought of as ironclad historical associations. While people almost certainly thought that Arabs were the descendants of Ishmael, this shouldn't be taken to mean that this was the result of either historical inquiry, theology, or demographics--they were simply associations of contemporary groups with biblical characters.