When Moses says "Now I know that Man is nothing, which thing I never had supposed" he is referring what he just experienced. Moses got to experience, to a small degree, what it is like to have the light of Christ proceeding forth from him to fill the immensity of space. That light of Christ, which is the same thing that we refer to as God's glory, is in and through all things and is the means by which God has power over all things, by which he governs all things, and by which he is in and through all things past, present, and future.
When Moses 1 says the glory of God was upon Moses, it was saying that Moses got to experience what that was like to a degree. He got to see things a bit as God can see them, through God's glory.
He then knew without any doubt that he could not be exalted in his mortal body. His body lost its natural strength for many hours despite God transfiguring him for a time to endure a portion of God's glory. Moses knew, with perfect clarity, that if he wanted to be exalted, to become like his Father in Heaven, he needed a better body. He needed a glorified body like the Father enjoyed.
And it is that knowledge he is referring to when he says "Now I know that man is nothing, which thing I never had supposed".