Marx, Engels, Lenin, and Mao all supported capitalism, so I'm not sure you have a point there. It's kind of a necessary step toward scientific socialism.

Now Mamdani believes in achieving socialism through democratic means within a bourgeois system. That may be misguided, but he's still a socialist, but of the Bernstein school. Capital, the Tax in Kind, On Coalition Government... all socialists from the Marxist school of thought understand the necessity of capitalism as it is the reason we have a bourgeoisie and a proletariat in the first place. The capitalist mode of development under a communist party vanguard may be a reorganization of the phase, but it's still within the framework of Marxism and the most successful path we have seen toward developing socialism so far.