Substantial non-duality is not merely the I AM stage. The I AM stage is what I call partial duality, this covers Stage 1 and 2, since even though the sheer presence is experienced as non-dual, nevertheless one views the world in terms of an ultimate observer experiencing the world, or the world being an appearance arising 'in' the observing awareness. Substantial non-duality is stage 4 where awareness is not seen as a container for appearance, but as something appearing 'as' phenomena, therefore phenomena isn't 'in' but is 'as' awareness. This is still substantial because one still reifies awareness as some inherent seamless field of oneness appearing as everything.
There is an article I wrote, Substantial and Insubstantial Non-duality