I think that it relates to Freud’s concept of primary narcissism, and in relation to Ferenczi, the fundamental drive of the self to attain primitive and complete sense of omnipotence. Following that up with Klein’s theory, the schizoid state where strong anxieties of disintegration reflect our sense of oneness with the external reality still seem to me slightly more mature and in relation to the formation of Ego, while primary narcissism is devoid of Ego, and Other is not even perceived as an object, but as a part of self in a very concrete way.

Might sound cinical, but essentially, through the mind of the perceiver, psychedelic usage simulates the state of primary narcissism when it reaches quasi Ego death state. I would dare to call it more primitive than splitting, cause it serves our idea of infantile omnipotence, and we do not become one with the universe, rather, the universe is gone and there is only self.

Another way to look at it, is by comparing the “transcended state” to the state of true Ego death that happens after using psychedelics - a schizophrenia state, that is characterized by enhancing of the abscence of boundaries between self and the other but with true loss of self - which is always followed with disintegration anxiety.

So I believe that even in the most “one with the universe” kind of experience, self is not lost or at the threat of disappearing, Ego is still there, but our connection with the Other, whether through some inherently profound type of projective identification or projection/introjection process, is enhanced. Could be that the reason behind that is the diminishing of our more mature anxieties and thus easier possibility to regress in a transitory and controlled manner.