I don't know if you are just interpreting from the Pilot, without taking into regard what we learn later

I don't think it's possible to exclude everything we know from three years of canon, but I'm trying to go by just what's in each episode.

But from the Christmas ep, we know that's when Sam learned about John hunting. I don't think it's true to say Sam knew nothing before that. Certainly both John and Dean did their best to hide it from him, but I think it's very unlikely they succeeded. What we saw in the Christmas ep was a Sam who had lived with the lies for too long; he must have known long before that.