If I may give my opinion, I must say that I find this reasoning unsound. In the example given, "if not more" is an addition which is not at all implicit in "as many books as". The reason why the addition does make sense is that the person who is making the comparison is not completely sure about whether Tom has as many books as John, or whether he has more books than John. I mean, the person in question first says that the number of books is equal, but then he realizes that this may not be the case because the number of Tom's books may be greater than of John's books.
Sextus