I don't so much think he tossed out his outline, I just don't think there was one to begin with. He comes from a TV background first and I think he had a great concept, great characters and had a general thrust of what he wanted to happen, that is a united kingdom thrown into war after the king dies with the specter of limited resources because of the winter coming and the true heir to the throne amassing an army in another country. As a concept it works and could drive a TV series, comic series or hell, even a series of books seemingly forever (if the author just wrote the damn things).
To me I never understood the authors who just wrote from the hip. Sometimes the books come out good, most of the time they are a meandering mess then the book ends.
I'm of the Victor Hugo, Arthur Conan Doyle, Ayn Rand, Ian Fleming, John Gardner, Tom Clancy, Jeffrey Deaver mindset where you plot, outline, research, figure out all the mechanics of the story, then write. Deaver has been known to have 150 outlines that condense the outline, research, character sketches into one piece, yet his books come out to be about 300 pages. Thing is, everything works perfectly. It's also the method most mystery and thriller writers use. He's been known to go through this process for some 6 months to make sure everything works, but the actual writing of the book is just a few weeks, enough time to write it out in long form and add the dialog.