They will behave like flat earthers and creationists who will consistently deny any relationship between communism and atheism even if presented with evidence to support that view because they will assert "they" can only be the judge of whether a relationship exists.
All of the atheists I know are atheists because we place a high value on evidence and we're skeptical of dogma. You don't have to hold those values to be an atheist, but most of us do.
The communist leaders were big on dogma. That's a crucial difference.
I suppose that if you're obsessed with using the term "atheist" to describe the communists, then we ought to have two terms that indicate the crucial differences between the communist atheists and today's atheists. Maybe we should just call today's atheists "modern atheists"? I'm not married to any particular terms, but the distinction is crucial.
I've heard some atheist suggest that we call ourselves "critical thinkers", but of course (and understandably), many theists find that offensive.
As our friend Confucius says, we ought to use proper labels for things.