It's not exclusively leftcom opinion but just a take that anyone reading Marx would automatically come into, especially first 3 chapters of Capital and Critique of Gotha Programme (leftcoms are theorycels though so they do come to this conclusion). You can't have commodity production and somehow be socialist, despite Stalin's falsifications.
It also goes against materialism into the realm of idealism, where a DOTP can look inwards and start pursuing its national interests to build itself up and compete within the capitalist world against other capitalist powers using capitalism and everything that it implies (like class relations), and somehow through magic still retain proletarian character despite it actively going against the interests it pursues and literally being the kind of society the proletariat as a class have the historic task of abolishing. This is nonsense, and that's why the primary objective of DOTP is internationalism and it's where it lives or dies, rather than indefinite national development as it's not a national project.
I tried to keep this as short as possible since I could yap about this for super long so it's not all-encompassing, though I'd also recommend reading Engels Principles of Communism and Bordiga's Dialogue with Stalin, the latter text is especially related to this topic.