When a wiki is configured as "private", Index is not reachable from any page but those which are allowed. So, it's very difficult (to not say "impossible") for users to find allowed pages without the ability of listing them.
I'm not sure I understand what this is about. Are you using the sneaky_index function? Then disable it. If not, please explain what you are asking for.
I think what he means is:
When your acl rules are e.g. "* @ALL 0", the index always shows the "Permission denied" page.
When you have restricted only e.g. "private:* @ALL 0" and you are on of those denied private pages, want to get out by clicking the index, you'll get the "Permission denied" again.
I agree the index should be independent from the page you're looking at and shouldn't follow the acl rules; it should behave as one single page with the same behaviour, regardless of the namespace you're in. It should still follow the acl rules when it comes to showing sub pages or not, and it should still unfold the namespace you're in (if you have permission to see its content).