Fordham Sports talking complete horse poo. Warranties apply from the moment you buy a product. A manufacturer can choose to state that direct damage to the toe or edge caused by lack of proper knocking in could void a warranty, but not that the warranty only applies 15 days post purchase.
For the first four-five weeks you have a "right of rejection" - if the item you've bought breaks, you can demand a refund, but in the case of a cricket bat with obvious caveats.