>i guess the form method POST is based on the http-referer, if i'm not mistaken
I'm mistaken, both the login form and the reply form use the method POST, but the login stops working without http-referer, while the reply form works fine even without. So the method "post" is not reliant on http-referer to work.
So i guess, who programmed this board, had decided to specifically check the http-referer exclusively during the login. And not elsewhere in the site.

Sorry for the false alarm btw, I'm just finding this curious the more i look into it.