I'd argue that it isn't. The "clean room" example is a measure of obviousness: if there is clearly one way to solve a problem, and someone can figure it out simply by being told to produce the outputs, I'd argue that it's obvious.
Now, that's not at all what happened with the MPEG_LA. VP8 (or WebM, or whatever) has a ton in common with H.264, which isn't independently likely or obvious given the mature state of the video compression field.