I guess she means he left communism. A belief in history, almost as a god, is inherent in communism as I understand it. The changes they believe in are pre-ordained, they need only to join the stream.
https://news.ycombinator.com/item?id=15367489
https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme...
So you're right about the association of historicism/Whig history with communism, but for Arendt that category (historicist ideologies) didn't just include communism.
It has a recognition that the forces of production change. That relations of production change, and this base changes superstructure in the society. Hunter gatherer tribes make way for Sumerian and Greek slave empires, make way for feudal Europe, which make way for capitalism and bourgeois republics.
The Paris commune, the Russian revolution, the 1936 Spanish republic etc. are seen in this light as nascent proto-steps toward the next superstructure coming out of changed forces of production and relations of production.
History has already seen four major changes from one system to another, but not by means of divinity. Marx's pointed to contradictions inherent to capitalism. I think the 2008 bailout and crisis would be a sign of the eventual end in Marx's view - US Treasury secretary Hank Paulson said GE CEO Jeff Immelt visited him on September 15, 2008 and said GE was having trouble financing day-to-day operations. Events like that are harbingers of the eventual collapse of the capitalist economic system, in Marx's view. For now, the government and taxpayer bailing out the capitalists and corporations work, in Marx's view, one day it will not work. But the collapses are only part of the collapse, advances had to have happened to shift the system as well. It is not divinity, it is history continuing to be dialectic.