There can be multiple reasons:
- Commenter just had a slip up and thought of it as a masculine profession which is fine. It does happen.
- Commenter's first language is not English. Czech for example doesn't use they/them for a single person unless it is neo-pronouns. So instead we just gender based on word of a profession, which is 99% of the time masculine. Only exception that comes to mind is a nurse which translates to homonym of sister. There are also feminine suffixes for jobs but that is only used if you know that the person who works it uses feminine pronouns or if the field is heavily dominated by women but then it is a prejudice.