Doing this will not allow you to view the other user's Friends-Only entries, nor will it allow you to be added to their friends list in return.
What does the second part of this sentence mean? There are two ways I can interpret this. It could mean: You cannot add yourself to someone else's friends list without their consent. But that's a more general fact and not specific to syndicated feeds, so I don't see why it would fit here.
The other way can interpret it is to mean: The person whose journal you are adding does not have a way of adding you back... which is clearly false, because we're talking about users of other LiveJournal-code based sites here. Of course they can add us back, by the same syndication mechanism as we added them. (The first part of the sentence has already established that access to friends-only entries does not work this way.)