I knew it was gonna be a girl the entire time so I was trying to think of a way the translators could have referred to the demon king without it being (even more) obvious what was coming.
I haven't looked at the raws so i'm not sure if it's the case here, but I know that often in situations like this the way they refer to the demon king is very direct, usually calling them 'maou' without using any pronouns, which makes it very hard to translate while keeping the intended surprise factor, but without having the characters use entirely incorrect pronouns.
Because in English it would be so rare to refer to someone so many times without using a pronoun, if you translate it as the characters always saying 'the demon king/lord' readers will immediately sniff out what's coming. There's a similar problem with if you use 'they', not having a gendered pronoun in the translation will almost certainly give away the "big reveal" (even if it's not really a big reveal if you read enough isekai manga).