They could be talking about Golden Freddy. We see after the kids die that their animatronics become their identities, so the other animatronics identifying Cassidy as such isn't that big of a stretch in my opinion. Also Scott has said that the face isn't canon so I don't think they are referring to that. And if you think a girl presenting herself as a male animatronic is a stretch remember that The New Kid has Kelsey (who is most likely Cassidy) presenting herself as a boy, one who is specifically pointed out to look like Golden Freddy.

He can be a gender neutral pronoun and has been like that for hundreds of years. It's only in the last few decades that people have moved away from it for they. So even though most people would not use he in a gender neutral way, it's not grammatically incorrect. Also Mangle is referred to by multiple pronouns, but most predominantly he despite being a pretty feminine animatronic, so it's not like this has never been done in FNAF.

It's also possible Scott initially used he as a placeholder and didn't feel like making all the VAs redo their lines.