It is two sessions per day. First the heavy weights then the lighter guys. Each guy gets 3hrs of training per day...
I think you are wrong about the intensity of their training.
I've studied BJJ at one of the Gracie schools... We spent 80% of the class laying down or sitting. We did train hard but got to rest in between rolling, and 90% of the sparring was done on the mat, intense but you are laying down the whole time.
These guys have 3 hours of intense training! punching, kicking, grappiling, learning takedowns... Throw in a 30 RUN... not a jog... A RUN! and you have a hell of a day ahead of you!
-BRR