That's an old warning and should probably be removed by AVS. It dates back several years to a time when Costco would replace or refund a projector no questions asked and with no time limitations.
While this was a great policy it unfortunately lead to abuse by members who saw it was an opportunity to try new projectors and return them after a few months or few years. They would take the refunded money and buy another new projector they were interested in. In other words they used Costco to upgrade their projectors for free whenever they thought something better had come along. They would then come on the AVS forum (and other projector forums) and brag about their latest abuse and spread the word that it was perfectly okay since Costco's return policy allowed them to do it - hence the eventual AVS warning. With time Costco became aware of the abuse and changed their return policy to what it is today.