I guess this implies that Facebook does not really remove the imported information, but that it keeps this information and just hides those contacts from you in the friend finder interface.
I think you're leaping to a bad conclusion. Keith's post mentions "computer-generated guesses" to fill in the list. Looking at the guesses in my list, they appear to be friends of friends. (The prefill list is probably using logic similar or identical to the logic that presents "people you may know.")
Find somebody on your mystery list who isn't a friend of your friends, and then you can get paranoid.
I see plenty of people I know of but have never interacted with online (in fact I just found the previous-unfound profile of a coworker with a very common name). Facebook is /smart/--though I wouldn't doubt for a second that they've used your friend finder data to strength their edgerank data I would not be surprised to learn their seeming precognition comes from more esoteric sources.
However, I've removed all imported friend finder contacts on https://www.facebook.com/invite_history.php some weeks ago.
I guess this implies that Facebook does not really remove the imported information, but that it keeps this information and just hides those contacts from you in the friend finder interface.