There is experiencing without reflection and then there is the experiencing that is reflection on experiencing, or the self-conscious awareness of experiencing. The latter is taken to immediately imply that there is an experiencing substance, that the 'I' is that substance, or is an instance of that substance. What a great number of tacitly assumed premises there are in that deduction - because the cogito can only be a deduction, and not an intuition or immediate perception. These premises all belong to the pragmatic heuristics of getting about in a world of already constituted objects ('where there's smoke there's fire' etc) Something is accomplished in that thought experiment, but it is not a piece of metaphysics, it is simply the sudden transition to self-consciousness, a self consciousness that remains to be understood on its own terms.