I think the point of the article was the conclusion that contrary to common perception, the sources of the fake reviews came from actual customers, not from competitors.
many of the registered customers, actual customers, were writing negative reviews online for products they had never purchased
The way that's written leaves space for something, it's purely technical:
You can be a registered user/customer on a site
The company could see that you NEVER bought the product you complain about
That could mean you didn't buy that product
That could ALSO mean you didn't buy that product THERE
I've seen people who are mad, feel insulted or ripped off and take any chance they see to talk in all fairness: bad about the product they bought (and yes, also TOO MAD about the product, more like an emotional reaction), I mean they want to spread the word everywhere they can.