What Kegan is describing in order 0 is a state where the infant does not have a bound identification, i.e. does not have an identification: she perceives, but doesn't categorise into me/not-me. That's not a sense of self, but just a sense. The "everything sensed is taken to be an extension of an infant" part is a descriptions of how we perceive the infant in that state, not a description of the infant's own reflection.

Also, why do you say that the theory builds upon this? I didn't find it to be an important part of the theory.

I didn’t get that from him. In state 0 the baby has sense but not about the self. Imo self is something taught by the society and nurturing so baby would not have that concept.

I see nothing wrong with it. Theory was not built on the idea of sense of self of a newborn. It was built on senses of self of people of older ages. Then the succession of selves was extrapolated to a newborn. Kegan got a degenerate trivial case, nothing wrong here.

Are you saying that there might one day be evidence for the nonexistence of a sense of self in an infant? I can't even find that kind of evidence for a rock--seems to me that Kegan is safe here.

Came here to say the same thing. In Westworld the sense of self is the very last thing to develop in Arnold’s Pyramid/Maze theory.