This is the truth right here. It is clearly defined in the policy of Capital One concerning extending the warranty, and I'd suggest that other credit cards are likely the same; make sure you do your research people so you aren't expecting coverage you aren't likely to get.
Here is the verbiage in question that outlines my conclusion:
First, from Capital One:
How does Capital One define an original manufacturer's warranty? As follows:Extended Warranty
When you use your Capital One card to purchase most personal items (and the full cost of the item is charged to your card), Extended Warranty coverage automatically doubles the original manufacturer’s warranty for up to two years.
The important bit is right here: excluding any extended warranty offered by the manufacturer or any third party. A third party in this case would be Costco.Original manufacturer's warranty means an express written warranty valid in Canada and issued by the original manufacturer of personal property, excluding any extended warranty offered by the manufacturer or any third party.
Next, from Costco:
So, if you trace this back: Costco extends the manufacturer's warranty to 2 years. As noted in our first definition above, Capital One automatically doubles the original manufacturer's warranty (defined to be 1 year, as outlined in our second definition above), for up to 2 years. Thus it can be concluded that both Capital One's extension of the original warranty and Costco's extension of the original warranty cover the same period of time, resulting in a total of 2 years of warranty coverage.2nd YEAR WARRANTY
Costco extends the manufacturer's warranty on televisions, projectors, major appliances and computers (excluding tablets) to 2 years from the date of purchase if the manufacturer's warranty is less than 2 years.