I am with the author on how people get radicalized, but he could not have picked a worse example that Marxism as an example of extremist idea.

Equating Marxism to labor theory of value is akin to maybe equating economics to a supply/demand curve.

There are unequivocally true parts of Marxism, such as surplus value of labor, and the fundamental incentive conflict between a capital owner and laborer (which Marx deems exploitation)

The labor theory of value is also I think misinterpreted by modern economists as a price-setting mechanism, which it isn't. It's a mechanism to show that there is a certain floor on value of an economic good, which is raised every time constructive labor is applied to a product.