Wow, that's cool. Certainly looks like a biface and rather a big one for quartz. It could be a preform, but I'll go out on a limb and say the guy knapping it knew he was getting close to having that piece of quartz fall apart or snap and stopped there to use it as a hand held knife.
For those having trouble seeing it, I'll mention that I know enough about where it was found that all the quartz there is in cobble form, so just finding a piece with no cortex on it is a major clue it was worked.