Is it not just that you're already logged in, the site has remembered who you are by the medium of cookies? What happens if you just post a message on a page where there's no login form? Do you get an error?
You know you can't actually post a new message here, only a reply? I see in one of your screen shots there you have tried to, that's what the Guests cannot start new threads here message is all about.
Cheers for going to the trouble of posting screen grabs and stuff!
💬 Testing
Paul Clarke's weblog - I live in A small town. Married + father to two, I am a full-stack web engineer, + I do js / Node, some ruby, other languages etc. I like pubs, running, eating, home automation and other diy jiggery-pokery, history, genealogy, Television, squirrels, pirates, lego, + TIME TRAVEL.