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It's just something stupid that people say when they've been caught over-generalizing.

It is a very old saying from when "prove" meant what "test" means now. Exception is meant in the sense of exceptional, unusual. "The unusual event tests the heuristic."

It's actually a really simple thing that most people misunderstand (such as the parent). It means that if you only provide an exception, it implies a rule. For example, if you say:

Parking allowed 3pm-5pm

that implies that parking is not allowed at other times.

Pointing out how flexible Haskell's type system is just proves how inflexible type systems generally are. It doesn't really demonstrate that most type systems are useful.