Claiming that the author is sexist for using "he" in a gender neutral setting, which has been standard practice for decades, is an over-reach.
Is "they" or "him/her" a better way forward? Sure. Is it standard practice? No. Can you infer anything about the author's attitudes by his use of standard English grammar? No.
Is "they" or "him/her" a better way forward? Sure. Is it standard practice? No. Can you infer anything about the author's attitudes by his use of standard English grammar? No.