Joe Louis Defense?

I am a big Fan of Joe Louis, however there is an aspect of his stance I have never understood. With the rear hand held near chest level, this facilitates parrying jabs, but doesn't this leave one dangerously exposed to left hooks? aside from ducking every shot how does one Proactively defend against the hook in this stance? For example it seems that this stance would be much easier to attack with the hook off of the jab since the rear hands default position is not held at the right cheek? Thank you to Lee Wylie for posting that awesome video. I greatly appreciate any and all info, thank you