I always find that characterization of Grey and the Cortex podcast to be weird. He never claims to be a productivity master or the most productive person around. Quite the opposite, he has said multiple times how much he is not naturally productive, and how he actually kinda dislikes working in general. The systems and habits are the ways he found to essentially trick himself into working.

Which I think is what people gather from him, but somehow think he's hiding it or pretending is not the case? Which I find strange, given how openly he's talked about it.

As for his productivity going down over time, I think that's a combination of his videos getting bigger scopes and production values, and also he moving some of his time into some not so publicly visible ventures. E.g., he was one of the founders of Standard, which eventually became the Nebula streaming service (though he left quite a while ago now).