The pronoun in the English language for a person of indeterminate gender is "he". Yes, it's spelled the same way as the pronoun for a person of specifically male gender; it's far from the only case of different words being spelled the same way.

This may be true according to official grammatical rules, but it isn't true for the purposes of conveying an understanding. When I hear the word "he" used with no other identifying characteristics, I don't imagine a person of indeterminate gender. Even if I'm actively trying to avoid assigning a gender to the person, my immediate reaction is to imagine a man, because that's what the word "he" usually means when I see it in writing. My conscious control means that I can afterwards respond as if the person is ungendered, but my mental impression is of a male.

For most people, reading and comprehending text has become an automatic process. It can't be easily controlled by an order from the conscious mind to "always interpret pronoun 'he' as if gender indeterminate when in X context." I expect most people don't even try, because it isn't worth the mental effort.

I generally get around this problem by using the singular "they," which I expect will eventually become accepted by the grammatical establishment, since it's already becoming common in colloquial speech. And then for cases where "they" is awkward, I simply alternate between using "he" and "she." Gender neutral pronouns like "ze" and "ey" are an option too, but parsing them can also take too much mental effort for some readers and break the flow, so I usually don't use them.