Homestuck, the comics story, is cyclical, and everything just sort of repeats endlessly. There is never actually an end to this cycle, even after the characters walk through that door, the old loop they made continues to happen outside of it forever, like a weird, shitty ouroboros. Because of this, I've always thought of these terms "Dubiously Canon" and "Post Canon" etc, to be more labels for "when does when part of these characters' stories happen."

When does Jane become a Fascist? In "Post Canon".

When do Roxy and John get married? In "Post Canon".

I think it's easier to just distinguish it in this way rather than trying to neatly define every single thing we see on screen as "canon" or not. In fact, I would even go so far as to say that because it has the word "Canon" in it to describe it, that means that the event did happen more or less, but how relevant that is to the old Cycle (The Comic) is something to be discussed. I liked that quote from Marvus that someone from here pointed out earlier because he is also just parroting what Calliope said about how even though nothing that happens is "canon" in one timeline, that doesn't mean it's not relevant.

To me, they are both speaking in a literal sense, that Canon and what's "Dubiously Canon" are both canon, but in different ways, that doesn't make one or the other lesser than. This is par for the course since it is Paradox Space, which allows for things like this to even happen. I think where people get caught up is that they view both Canon, Dubiously Canon, and Post Canon not as equals, but as quantifiable things that they can measure on a pedestal or scale. In actuality, rather than them all being "true," it's that they're all equally as important to the story being told to us, the audience of people who read the comic to begin with. And we influence this universe just as much as anyone else would.

I know someone mentioned Octopimp, but even now, I still see people say they aren't exactly happy about the voices for the animated pilot because Octopimp's voices are just what they hear whenever they read the comic. Some people are saying they can't stand it because the Vox dub of the comic is the only interpretation of the characters' voices that they like. Hell, some people genuinely hate Karkat's dub voice because all they can associate with his voice is Broadway Karkat. Now, what exists are two versions of the characters' voices, but the vast majority of people dislike the "canon" interpretation of these voices. Does this then mean that the "Non-Canon" voices are no longer important or equally as respectable as the official ones? Obviously not, they are both important in their own unique way. They just originated from different points, one is purely "fanon" and the other "canon".

So to reiterate my point, I think trying to define what's canon as "this is what happened and is important to the main story" largely ignores that a lot of Homestuck puts both "fanon" and "canon" on the same pedestal, and because of the amount of fanworks and fan contributions over the years, what is 'dubiously canon' is also just on that same wavelength. They're all canon, but just come from different sources. Whether or not you think some of these sources are irrelevant is subjective IMO.