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?”
Because Socialist Market Economy refers to the whole economy, in all its working organizations and policies.
And NEP is a specific policy of market liberalization, which, with resources, investment, and productive forces of industry and agriculture, help develop and keep up to speed the USSR’s socialist economy, during such period where it was left with semi feudal economy and the Civil War ruins