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

Just a few of my cats are male. This proves that all my cats are female. Right?

I think he's taking as axiomatic that there only a few people using ruby, thus claiming this demonstrates the general rule people don't use ruby. Thus he is, in fact, using the phrase correctly.

But by taking it as axiomatic that few people use ruby, he is begging the question.

It's just another hand-waved argument in the whole confused post. If the dude's approach to code matches his approach to writing then we have nothing to worry about.

The most charitable interpretation of the claim I could think of was that the author meant the few exceptions illustrate why the other companies are moving away from ruby.