The meaning of ‘the rule’ in the context

The following passage is from the website as follows:

http://www.epubbud.com/read.php?g=H8AVF5J5&tocp=7

The story is not quite that simple, because the Tasmanians did invent a few new things during their isolation. Around 4,000 years ago they came up with a horribly unreliable form of canoe-raft, made of bundles of rushes and either paddled by men or pushed by swimming women (!), which enabled them to reach offshore islets to harvest birds and seals. The raft would become waterlogged and disintegrate or sink after a few hours, so it was no good for re-establishing contact with the mainland. As far as innovation goes, it was so unsatisfactory that it almost counts as an exception to prove the rule.

In this paragraph, I’m not sure what the underlined ‘the rule’ means in the context. I think ‘the rule’ seems to refer to ‘According to the anthropologist Joe Henrich,...’ three paragraphs before. But even if I am right that still doesn’t make any sense because that doesn’t show how could the coarseness of canoe-raft made by isolated Tasmanians be considered as an exception to prove the rule.

Can you give me your thought, so that my blurry brain can be cleared caused by this tricky question? (To do that I think you should read at least four paragraphs and I am sorry to trouble you for all this fuss I bring on.)