The language above is a notice to people who haven't purchased yet... that the offer being shown is subject to modification at any time. But once you've paid your money, the contract is sealed with whatever terms were in effect when payment was exchanged. If Disney quietly removes the "no blockout dates" language from the page and you buy after the change, they can point to the language that it was "subject to change" if you complain.
Disney has a very strong legal department to avoid class action lawsuits. So, if they advertise "no blockout dates" I would accept that as a contract. They don't want to be paying settlements to the swarms of lawyers who are looking for a big corporation to trip up. Just as, if you booked your on-site room when Disney was advertising "free overnight parking" you could rest assured that when that policy changed, the old "free" language still applied to you. And when I redeem my old 1990's era "all three parks" hopper ticket that I found in my parent's garage, they will honor that for at least every park except Animal Kingdom.