The Octopus Mastercard has been around for a few years. It's just a reloadable prepaid Mastercard that can be funded from your Octopus Wallet. Visa/Mastercard debit cards were quite rare in HK until recently, so I'd imagine it was originally aimed at people in HK who were unable to obtain traditional credit cards but who still wanted to buy stuff that could only be paid for with a credit card (like a lot of things online).
I believe that, aside from being against the terms of the Consumption Voucher Scheme, there are some actual barriers to transferring your Consumption Voucher Scheme money to your Octopus Wallet and then on to this Octopus Mastercard.