It mentions "Because you have complete control over communities that you have posting access in and the accounts you list as friends, LiveJournal does not allow you to hide these lists"
Listing as friends is simple enough, and makes sense. Even if you could hide people you would list as friends it wouldn't make sense since people could see it looking at your friends' page. It also fits with certain communities' stipulation that to remain a member you need to list the community as a friend.
That we have complete control over communities that you have posting access in isn't exactly true though, and does not fit with the 'how to hide communities' option. Since anyone can just view the 'full' mode of a user's profile view, the guide becomes pretty useless, as people could look there. The only communities this guide actually helps you hide are ones you don't have posting access in.
As far as I know, the only people who can control posting access are the moderators of a community. So ironically enough, you can only hide the communities you moderate, and not the ones you're just a member of.
That or... is there a way to remove your own posting access without bothering the moderators to do it for you?
Or... maybe just don't make it public what communities you have access to? It's not really relevant to a profile. The whole idea of a friends' list is that you're listing the communities you want people to see. Creating a whole other journal is also troublesome and it complicates the LJ system to have more accounts than necessary.
I realize it's possible to remove your own posting access by removing your membership, but should it come to that? Many communities that don't have automatic posting access for members and that allow it manually have people who are members of the community and who can be members without requesting posting access. This allows people to keep a list of communities they're interested in by becoming a member, but they wouldn't have the privacy exposed by the posting access list until they decided they actually WANTED to post.