Not official. People are free to write whatever they want anywhere on the Internet.
The downside is, instead of contributing to the official wiki so we get a good and complete wiki, we get a bunch of unfinished, incomplete, duplicated, deprecated, non-curated wikies everywhere while the official one lacks contribution.
This one is the most obnoxious, because it says it is "official", lol: https://galacticraft.mods.wiki/wiki/Galacticraft
It's also copying text from our actual official wiki (from a version about 4 years old). That's copyright text, so actually copying it like this is really not permitted.