Not OP, and a non-native English speaker, even though English has been my first/primary language for a while now.

I also referred to Murderbot as "he" in my first review (before I realized what its preferred pronoun is). I did not assign a gender to Murderbot though - I thought of it as a genderless entity, if I thought about its gender at all. I just tend to use "he" as the default pronoun without thinking. My mother tongue doesn't have gendered pronouns, and I have never really gotten used to thinking of gender identity before using a pronoun. In my mother tongue masculine words also generally double as neutral gender words (similar to old English where man = human of any gender, and wif-men = wife-men/women/men of the female gender). So I just tend to default to male words in English too, unless I'm consciously making an effort.