Sundance is a Marxist

(Sundance is the proprietor of a prominent blog

"The Conservative Treehouse"

theconservativetreehouse.com )

In what way is the assertion in the subject line true?

Because he constantly pushes the idea that powerful people (descriptors he uses include "the Davos set", the WEF, and in America, "the Sea Island crowd", 

use the control over society they undeniably have

for strictly economic reasons, 

to improve and protect their economic status.

What does this have to do with Karl Marx?

Sundance's theory may be described as <I>economic determinism</I>.

You can read about it at

https://study.com/academy/lesson/economic-determinism-and-karl-marx-definition-history.html

"Economic determinism is the social and economic idea that 

all parts of a society or culture 

are created as a result of economic processes and systems. 

This political, social, and economic theory 

was developed by Karl Marx as he and Fredrich Engels developed the central tenets of Marxism."

See also 

https://en.wikipedia.org/wiki/Economic_determinism

Sundance probably doesn't fit all the characteristics of being a Marxist, 

but he does share the economic determinism one.