Websites make up Winds of Winter rumors as clickbait all the time. October 2015 is already pretty much impossible.
On the other hand, Martin did make an intriguing aside in a blog post tonight. After announcing that he wouldn't be attending this year's San Diego Comic-Con (July) or World Fantasy Convention (November) due to various professional commitments including TWOW, he added "(Should I complete and deliver WINDS OF WINTER before these cons roll round, I reserve the right to change my mind)." I wouldn't read too much into that-- he felt like he was close to delivering ADWD in 2009, and then it ended up taking two more years-- but it at least indicates that his finishing the book this year isn't laughably implausible.
Pretty sure HBO already knows that further contract extensions would be an expensive proposition. But GOT is the network's biggest overall success since The Sopranos; they're not going to be afraid to spend money on it. Ten seasons sounds like hyperbole, but I wouldn't be surprised if they tried hard to get Benioff and Weiss to go at least one year beyond their preferred seven.