Class conflict isn't all there is to Marxism. The problem with capitalism for Marx was fundamentally one of production for profit, in which the capitalist class is incentivized to extract as much surplus value as possible from the workers. Billionaires and wealthy capitalists aren't the real problem with capitalism- profit is. The billionaires only do what a system of production for profit incentivizes them to do. Mao himself called capitalism a system of "profits in command." It follows therefore that non-capitalist economic systems are systems where profits are not in command. China does not have a system where the laboring masses work only for the profits of its capitalist class, rather China uses a market sector and market dynamics, including the creation of a capitalist class for management, in order to solve the problem of poverty. The standard of living has consistently gone up in China since the period of Reform and Opening Up. Extreme poverty was eliminated last year. Industry in China is ultimately organized to serve the public good, not profit. Who cares if they made a few billionaires along the way?

In fact, a fringe benefit to allowing some people to get incredibly rich is that it keeps that kind of person out of the government. Actually existing socialist states had issues where opportunistic people who just wanted to be wealthy and successful would enter the party and the state, steal everything they could, and ideologically decay the party. It's probably a significant reason the USSR fell- the Nomenklatura elites wanted to get rich and they killed the socialist project. It might be better to just let some people become billionaires but marginalize them politically than give them the keys to state-owned industries where they can amass tremendous wealth and power, but teem with frustration that they can't have anything nicer than a luxurious apartment and a dacha.