I hope everyone is having a nice day. Long post incoming.
I see that the wiki is using she/her pronouns for Grelle, and I also see that the admins have decided to stop waiting for “official confirmation” on her gender. I wanted to give my two cents, which is that there already was “official confirmation” on Toboso’s blog, when Yana said Grelle was a man with a maiden’s heart. I’m not linking the blog post here because I’m guessing everyone is already familiar with it, but this is how Japanese speakers normally talk about transgender women.
If you want an idea of how extremely common it is for Japanese speakers to use this terminology when talking about trans women, you can copy and paste 心は女 (“heart is a woman/female”) into your search engine of choice. You’ll notice virtually every result is discussing trans people. Meanwhile, when Yana talks about Grelle in her post, she says 心は乙女 (literally, “[Grelle’s] heart is a maiden/girl”), which is just a more flowery version of the same thing.
Even though this kind of terminology is considered a little outdated by now, 心の性 (kokoro no sei, “sex of the heart”) is still more commonly used as a layperson’s term than the official Japanese term for gender identity, which is 性同一性 (sei douitsusei). That’s probably because it’s been around for a lot longer (people have been using this terminology for half a century) than the official term. Here are two examples of this terminology cropping which are accessible to English speakers:
Interview of the owner of an FtM bar in Tokyo with English subtitles (see 3:00)
A scene in One Piece that caused a lot of trouble in that fandom
To be clear, “heart is a woman/maiden” would never be used to describe someone who is merely a feminine cisgender man; you would actually say the opposite in those cases. For example, Felix Argyle, the little crossdressing catboy from Re:Zero, says this about himself in the Japanese web novel:
「そう、フェリちゃんは身も心も男にゃのです」
“Sou, Feri-chan wa mi mo kokoro mo otoko nya no desu”
“That’s right, Feli-chan is male in both body and heart”
Generally when Japanese speakers bring up the topic of Grelle’s gender identity, whether it’s the narration in a Pixiv fanfiction or in a discussion on Twitter, they echo these terms because Yana saying “Grelle’s heart is a girl” is already seen as confirmation. I was about to go to bed when writing this so I only pulled up a handful of examples of Japanese speakers talking about her this way, but if anyone asks for more I’ll be happy to supply some. If not, just trust me that these terms are commonly used and understood within the Japanese fandom.
Pixiv’s page for Grelle, which cites the character guide in the same sentence it claims her heart is female
Example of someone using the exact same phrase while examining statements she makes about her gender in the manga and anime
Another person explaining the different uses of “onee” using the same phrase to describe the difference between cis male crossdressers and gay men (whose hearts are male) compared to trans women
TL;DR, some of the language Yana uses to describe Grelle would never be used by Japanese speakers to describe a feminine cis man, because it refers to a trans woman.