iirc its less like "john going by june is now canon" and more "this is a spoiler that she transitions, but yall asked so here ya go" re June being canon. It is confirmed as going to be canon, and because of a lot of different discussions in how gender functions even when not presenting as such, it is also functionally currently canon, if just in a particular early point of that identity realization. "Beyond Canon" as a whole is sort of a metanarrative exercise in how the idea of "postcanon" has been structured imo. Because it's obviously "canon" in any meaningful sense, but part of it being by a different team (with AH's approval) does in itself become a "well if it's not by Andrew does it matter?" question, and calling it BC highlights that.

HS:BC is also not done so I think the solidification of June will be much more concrete once it's complete, but currently June is just a reveal of a spoiler (or maybe even just something that was a thought, but not Decided until it was brought up and someone went: wait that's awesome and totally makes sense, and they committed to it. who knows). One of AH's biggest strengths in writing even base HS was riffing off their own work and improvisation. Like the whole initial premise of responding and building a story off of user input is just a long form improv exercise, and using "Toblerone" wishes is a continuation of that. It's like saying that Dave Strider being his name isn't relevant canon because it was a user input. That's how Homestuck works.

For now I think most of the arguments around June arise come up between people really excited/already integrating it into meta analysis and then the people having a reflexive "uh, no!" because it hasn't been shown in comic yet. A lot of the initial denial was also paired with a decent amount of transphobic rhetoric so it's hard to detach negative feelings from that. Some of the current denial is also still paired with some weird rhetoric so June denialism tends to just read kind of weird.