Genuine question
Edit: change the title from “Why can Socialist Market Economy be categorized as socialism, and why is NEP categorized as state capitalism?” to “Why can Socialist Market Economy be categorized as socialism meanwhile NEP is categorized as state capitalism?”
The NEP was characterized as state capitalist to differentiate it against the lower stage communist stage that was supposed to come afterwards. Lower stage communism was a particular definition for socialism used by Lenin.
Lenin would not consider the “Socialist Market Economy” to not be “socialism proper” i.e, not lower stage communist. However, he may consider the SME to be socialist in the same way that the Union of Soviet Socialist Republics was during the NEP… that is, aspirationally socialist (willingness to realize Communism) or a proletarian dictatorship.
Reading: https://www.marxists.org/reference/archive/stalin/works/1928/12/28.htm