Obviously, the answer is no. Chivalry & Sorcery, RoleMaster, Bushido and even AD&D are not rules light systems. So, why is it that, in general at least, we think of the OSR as a "rules light movement?
I think the answer to that lies in the ages of those writing the clones and their derivatives. They try to capture (with much success) the feeling and magic of old school gaming while making the rules more digestible and presented in an easier to follow format. The charm of the original rules was not in their presentation, but in the style of play those rules encouraged.
That, and as we get older, KISS (keep it simple, stupid!) makes gaming easier, especially on the aging eyes ;)
AD&D wasn't necessarily as complicated as it now seems in retrospect, but the horrible organization of the rules spread over two books made it difficult to master. Even early editions of Tunnels & Trolls suffer from horrible organization of the rules, making what should be a simple system difficult to learn by the book.
Some thoughts that came to me while listening to the latest Save or Die Podcast (with special guest Jon Peterson)