"Be aware that at this time, the ban_set command described only prevents the person from commenting in a community; if he is a member in an open community, for example, he will still be able to post."
appears to have been false for some time now. Banning a user from a community does in fact prevent that user from posting in that community (even if it's open) or joining it.
Unless I'm missing something obvious here, this should be deleted and perhaps some words should be added to the effect that banning does prevent the bannee from posting/joining.