The Evoker's pronouns haven't changed - it's not a mistake. The Evoker is referred to with they/them pronouns in ToA because he's androgynous, so we (and Hiro) don't know which to use. It's not because he personally uses they/them pronouns (although, incidentally, the Evoker wouldn't care what pronouns you used for him/her/them - he has other concerns). I tend to use they/them pronouns for everyone either when their gender isn't known or occasionally to disambiguate pronouns in a scene where multiple characters share the same typical pronouns. ToA has non-binary (and GNC) characters, but doesn't have anyone who goes exclusively by they/them pronouns, and the Evoker has always considered themselves gay (with a little bi lean, similar to Persephone, who is primarily a lesbian but will also sleep with men - homoromantic, bisexual).