Homestuck spends a lot of time showing how strictly-defined categories don't actually work in life, and I think canon reflects that. Is Nic Cage being god tier canon? The correct answer is "sure." Somebody arguing that it's totally not canon looks about as absurd as the person trying to explain why it's important to Homestuck that he's the Nic of Time.

I mainly use "post-canon" as a word to explain that Homestuck the webcomic exists independently of the Epilogues and HS^2. It ended where it was meant to end; whatever loose threads existed were fully intentional. "Dubiously canon" rings a little too close to what Homestuck 2 (different from Homestuck^2) wanted to be for my ears. We already have terms like AU and canon-compliant. I don't think Homestuck needs a specific set of words.

I will note that Hiveswap was also written to be enjoyed by people who haven't read Homestuck, much like how Homestuck was written in a way that people who hadn't played Hiveswap would still understand what's going on.