That explaination doesn't consider that the brightness falls off with each bounce and distance travelled, as well as the colours of the objects it is bouncing off. (Bright colours more reflection. darker colours more absorption). There is also the initial intensity of the light.I'm not sure what area you mean, but I agree the screenshot doesn't look great on its own. The video shows that there is a lot of scattered light coming from the left, from an area in the ceiling, which then explains the greater amount of light in the comparrison. The original never bounced that light leaving it looking so dark. It feels quite natural playing through that area.
I don't even think that level of light scatter and brightness in the surronding area would be possible without a mirror being underneath the opening.
Edit: Also the orginal would be using pre baked lights. Which would have been calculated using ray tracing :|
I suspect the true physically correct image sits inbetween the two.