The T&C of the rate are pretty clear. It looks like they are trying to encourage early booking, but know that the market won't tolerate pre-paying or non-refundable reservations. In other words, you're benefiting from planning ahead because maybe most of their guests reserve much closer to their stays. I'd keep the screen shot or print out the confirmed reservation, but I have always found Marriott to be honorable when it comes to the published rules and conditions.