A Song of Ice and Fire is High Fantasy because it relies too much on playing with its genre. You can have deconstructions that highlight the obsurdity of a genre by changing the environment, but SoIaF doesn't do so that drastically.
SoIaF sets itself apart from its genre more in tone and plot than it does in setting.
Martin is an admitted Lord of the Rings fan (which means little more than when musicians from diverse metal subgenres all name the same proto-metal groups as their influences) and with the "end of an age" style of both works ASoIaF feels like it could just be middle earth progressed a few hundred years.