That’s how many people originating from FB/X/Insta understand it, because they are used to a “single brand, single platform” model. I have a lot of explaining on that topic.
But once you explain how it works - like “EC posts from EC instance, and you post from mastodon.com.pl instance, but YOU CAN INTERACT as if you were on one platform” they get it.
The discussion about what is forum or not is semantic - for me as an infosec guy ActivityPub is just a really clever way for websites to interact between themselves and I like the opportunities it opened 
In theory, even an old classic phpBB could have each forum post or topic shareable over ActivityPub, with external comments appearing in the thread too. The question is whether anyone implemented this yet and would it catch with them.
But that’s because they are used to delivering angry monologues which are then boosted by millions of bots. This naturally didn’t work here.
