In the manga, Hanji is non-binary; meaning Hanji is not assigned a gender role. [...]
Hajime Isayama has not tacked a gender or sex on them, and translators of the manga have been instructed not to do so as well.
So, if you want to be correct about it, you should be addressing Hanji with gender-neutral pronouns like they/them.
I just want to clarify: non-binary is different from unidentified gender, which is how Hanji is described both by Isayama and the Kodansha staff. Being non-binary would mean that Hanji does not identify as either of the two generally accepted binaries (male/female) but the mangaka asked the English Kodansha staff either to not assign Hanji gendered pronouns or to assign gendered pronouns in equal measure. "He" and "She" can both be used to identify Hanji, as long as both of those pronouns are used with the same frequency. That's pretty much in contradiction to what a non-binary is.
Hanji falls more on gender-fluid than non-binary. People calling Hanji "him" or "her" isn't wrong at all.