The question is
Is a proposition a priori if the premises require empirical evidence?
Jason S. Baehr describes a priori and a posteriori as "ways of knowing", but they can also be applied to propositions and arguments:
The a priori/a posteriori distinction is sometimes applied to things other than ways of knowing, for instance, to propositions and arguments. An a priori proposition is one that is knowable a priori and an a priori argument is one the premises of which are a priori propositions. Correspondingly, an a posteriori proposition is knowable a posteriori, while an a posteriori argument is one the premises of which are a posteriori propositions. (An argument is typically regarded as a posteriori if it is comprised of a combination of a priori and a posteriori premises.)
If one accepts this, then an argument containing premises that are knowable a posteriori, such as the one in the OP's question, would be classified as a posteriori.
Baehr, J S. A Priori and A Posteriori. Retrieved on June 25, 2019 from the Internet Encyclopedia of Philosophy at https://www.iep.utm.edu/apriori/