Well for one thing, such countries don’t have billionaires and and a stock exchange with private corporations like in Vietnam, China and, I am guessing if I looked, in the other countries you listed too.

Marx never once said that Communism could be established through fiat, by decree instead of degree. Engels, in writing The Principles of Communism, makes it quite clear why this cannot be:

Will it be possible for private property to be abolished at one stroke?

No, no more than existing forces of production can at one stroke be multiplied to the extent necessary for the creation of a communal society. In all probability, the proletarian revolution will transform existing society gradually and will be able to abolish private property only when the means of production are available in sufficient quantity.

Marx describes the process of private industry forming monopolist syndicates ripe for public planning in Manifesto of the Communist Party:

The essential conditions for the existence and for the sway of the bourgeois class is the formation and augmentation of capital; the condition for capital is wage-labour. Wage-labour rests exclusively on competition between the labourers. The advance of industry, whose involuntary promoter is the bourgeoisie, replaces the isolation of the labourers, due to competition, by the revolutionary combination, due to association. The development of Modern Industry, therefore, cuts from under its feet the very foundation on which the bourgeoisie produces and appropriates products. What the bourgeoisie therefore produces, above all, are its own grave-diggers.

Socialist states with minority private sectors are fully in line with Marxist analysis. As these private sectors monopolize and develop the productive forces through competition, they make themselves ripe for public planning. A good essay on the subject is Why Public Property? if you have need for further detail.

If a handful of people or just one person owns a bunch of factories, a few individuals control the means of production. And do it to gain capital.

Yes. In the private sectors of Socialist States, this indeed happens.

Someone remind me what an economy is called when there’s an elite group of wealthy people who get rich off of their capital gains…

Depends on the overall composition of the economy, and what class is in control. Cuba has 77% of the economy in the public sector, for example. Hardly sounds Capitalist to me. The PRC had 50% in 2012, and roughly a tenth in the cooperative sector, and the Public sector has only grown since then, and state power over the Private Sector has only increased with time.

Is your argument that an economy that is not fully socialized cannot be considered Socialist? I think, for similar reasons that you wouldn’t call the US Socialist for having the USPS, that that’s a silly argument. Is your argument that not having a fully socialized economy is against Marxism? I think the quotations have helped hammer that Marxism is about progression to socialization, rather than forcing socialization without the necessary infrastructure. Is your argument that Marxism isn’t Socialism? That’s a hard sell as well.