Because they're not declining candidates because of their gender. They're hiring from talent pools where both genders are more properly represented.

None of this stops a candidate from applying to duolingo directly. There's no discrimination going on against humans, and discriminating against a school is totally fine so long as there's no suspect reason to do so (e.g. discriminating against Howard because they've got primarily African-descended students).

Not a lawyer. Would welcome a legal opinion.