Agreed, I think the author has mis-understood what it means. Quite a common one to get wrong though, for anyone wondering it generally means that stating an exception proves/ establishes/ implies that a general rule exists.

E.g. You may not play ball in the park within 20 metres of the road implies that the general rules is that you can play ball elsewhere in the park.

http://en.wikipedia.org/wiki/Exception_that_proves_the_rule