Course with guest access enabled (either with or without access key) allows guest to see the "add a new discussion" button and click it.

Moreover, if a course access key is enabled, guest are are prompted they cannot post only after clicking the bution, then redirected again to the Guest access page an requested to enter the key again.

If guest re-enter the key, he is prompted "To perform that action you need to enrol in this course" and again re-directed to the access key page, in a loop. The loop stops only when guest navigate elsewhere.