Touhou's canon is mostly centered around "whatever ZUN writes", as he is the sole owner of the series and writes the vast majority of content for it. The issue comes in the form of secondary works; Touhou is a doujin series, so it is extremely lax on what people are allowed to create and sell based on Touhou. However, in order to be sold on storefronts like the app store, Nintendo E-Shop, Steam, or so on, a fanmade Touhou game has to be officially licensed first. This means there are hundreds of games we could technically consider tertiary canon, due to our rules on officially licensed works, but don't because it's a byproduct of Touhou's nature as a doujin series and is not the creator treating those works as semi-canon. So we're already kinda skirting by this site's tertiary canon rules, which is a bit confusing in and of itself (I recall at least a few people off-site saying Lost Word, a secondary work, should be usable on here, for example).
However, this is also not accounting for secondary works wherein ZUN contributes his own work; We currently allow things like Seasonal Dream Vision, which is a secondary work including a short manga written by ZUN. We're allowed to use the manga itself, but nothing outside of that. We could presumably use ZUN's contributions to other manga and games as well, should I find any more. This isn't that complicated by itself, but recently a Touhou TTRPG was announced, where ZUN will write various flavor text as well as a gameplay demonstration. This sort of messes with what we typically use for canon, because there's no clear line between what ZUN writes (a demonstration of the gameplay mechanics alongside a story scenario) and what he doesn't (the gameplay mechanics and story scenario themselves); The former cannot be used in a vacuum, because it's inherently using the latter as a part of itself. This is even further confused by how people close to ZUN aren't even sure if the work is actually a secondary work or not specifically because of ZUN's involvement. I'm currently leaning towards treating the entire thing as secondary canon, with the caveat that possible future supplements wouldn't be canon at all (unless ZUN contributes to those too).
I don't think anything on the canon page helps clarify what to do when something that's technically technically tertiary canon from a series that doesn't treat tertiary canon as canon but is also maybe not tertiary canon at all(???) and has direct involvement from the author, whose words, regardless of context, are treated as primary canon.