Basing their politics primarily on references to philosophers who died more than a century prior.

Domenico Losurdo, Michael Parenti, Assata Shakur, J. Sakai, Frantz Fanon, Antonio Gramsci, Roland Boer, Jones Manoel, Mao ZeDong, Xi Jinping, Deng Xiaoping, Chen Yun, Cheng Enfu, Li Shenming, Wang Weiguang, Hou Huiqin, Zhang Weiwei, Samir Amin, Walter Rodney, Vijay Prashad, Gabriel Rockhill, Zak Cope, John Bellamy Foster etc.

Foundational theory also clearly still applies unlike much of early genetics work:

Marx’s theory of surplus value, the value produced by labor still exceeds the wages paid to workers, resulting in profit for capitalists.

Marx’s theory of class struggle society is still shaped by antagonistic class interests.

Marx and Engels’ theory of the state, the state still remains in place protecting class rule and property relations.

Lenin’s theory of imperialism, monopoly capital, finance capital, export of capital, sanctions, debt, unequal exchange, and spheres of influence are still central to the world system.

Marx’s theory of capitalist crisis, capitalism still produces recurring crises, unemployment, overproduction, austerity, and financial instability.

Engels’ argument in On Authority, revolution, large-scale production, war, and state power cannot be handled through pure spontaneity or anti-organizational moralism.

Marx and Engels’ theory of ideology, ruling-class ideas still dominate media, education, culture, academia, and “common sense.”

Lenin’s theory of organization, capitalism is organized, armed, global, and disciplined, so serious opposition to it also requires organization, strategy, and discipline.

And so on...

Marxism is not mainly a list of old opinions; it is a method for studying society, class power, exploitation, imperialism, ideology, and historical change. In that sense it is less like treating Darwin or Mendel as the final word on genetics, and more like still learning Newtonian mechanics in physics. Newton was not the final word, but you do not understand physics by skipping the foundations.

Also, most people do not actually have a meaningful grasp of the foundational works in the first place. They have half-remembered summaries, liberal caricatures, or internet slogans. And Marxism has not been “superseded” as capitalism’s core relations remain intact across much of the world: wage labour, surplus value extraction, class rule, imperialism, and crisis. Much of the foundation is still clearly very relevant.

QinShiHuangsSchlong beat me to the punch, there are countless modern Marxists and Marxists since Lenin that have continued to apply the Marxist method to new eras and new conditions. Marxism-Leninism is referred to as an immortal science because it's based on an ever-adapting framework for understanding the world, dialectical materialism, which in all this time have proven adaptable and fundamentally correct. We may teach Marxism in a new way with new conditions as we discover new eras, but the baseline is still applicable and necessary.