I don't think that's correct. All it says is that if you do purchase prior to the cruise the discount will be applied onboard.
"Discount only available to Captain’s Circle Platinum and Elite guests, for MedallionNet® Wi-Fi packages on sailings embarking after
July 24, 2021. MedallionNet® Wi-Fi packages purchased pre-cruise will be discounted on board and the guest will receive a refund credit in their folio. Discount is not combinable with any other MedallionNet Wi-Fi offers, discounts, or promotions. Discount is not transferable and has no cash value."
So it would seem easier to wait until you are onboard to buy the package.