You are confusing them with the communists who tried to use WW II as a cover to take over in post WW II.
Remember the comunists tried to take Greece when people were at their most miserable.
Yes, of course, it was Greek Communists who murdered Eleni. I’m no expert on Greek history so forgive me if I get some of my facts wrong. But in my reading of the book, the horrors that befell Eleni and her family started long before the end of World War II.