Did you read the article? It specifically mentioned that firms/fake reviewers use Verified Purchases as a method of building authenticity and believability. Specifically:

"The compensated-review process is simple: Businesses paid to create dummy accounts purchase products from Amazon and write four- and five-star reviews. Buying the product makes it tougher for Amazon to police the reviews, because the reviews are in fact based on verified purchases."

It means Amazon facilitated sending the product which AFAIK also covers when companies "give" reviewers free product so I don't really trust those a whole lot more, maybe a little more but it's not perfect.

AFAIK companies can provide discount codes for reviewers to "buy" the product but there is a limit to the discount amount.