You are correct. Creators chose this tag, early on, not anyone from the Itch.io staff. Still, if there was a compelling argument, and sufficient community consensus, it wouldn't be a big deal to change the database so that "LGBT" is renamed to something more inclusive. The question is what. "LGBTQIA+" is unwieldy, arguably off-putting to some people, and probably still leaves a few letters out. "Queer" would be my favorite, but some people still think it's a slur (mistakenly, by the way; there's a whole story behind that, too). Other attempts at inventing a more general acronym failed entirely.
For better or worse, "LGBT" caught on because it's, well, catchy, and also reasonably short. Much like we're stuck using the term "game" for a lot of interactive works that don't really meet any dictionary definition thereof. People are welcome to try and change the situation, but frankly, that seems likely to take a while if it will happen at all.