If you actually read their statement, they're not voting because they like Biden and the democrats, but because they believe that Trump is so dangerous that he needs to be removed immediately.
This is an excerpt from their statement:
"And the Democratic Party has done this, time after time after time, while itself representing and seeking to preside over this system of capitalism-imperialism which has white supremacy, male supremacy, environmental plunder and wars for empire built into it, at the same time as the Democrats make noises about, and take some steps to implement, minor (and ultimately meaningless) reforms to supposedly address these outrages."
Notice how the communist party criticizes the dems for *not* being communist. If you read the statement in full, you can tell they they are EXTREMELY reluctant to vote for Biden but they need to out of absolute necessity, because in their view, Trump is an existential threat. They didn't decide to vote because Biden is a communist. That's stupid.
https://revcom.us/