While there may not be large numbers of card-carrying communists lining the halls of Congress, there is a clear tie between the Democratic Party's Congressional Progressive Caucus (CPC), Communist Party USA (CPUSA), and the Democratic Socialists of America (DSA).
It really boils down to marketing. In marketing, many times the same product is given a different name or label in order to increase its appeal to certain groups. Names are sometimes changed due to the product's connection to other products, or the public's association to a prior name.
Although Progressive share much in common with CPUSA and DSA, they are shrewd enough to understand the terms "communist" or "socialist" are unpalatable for most Americans. Hence, the word "Progressive" was injected into American political verbiage. While the words are not interchangeable, one thing is for sure: The CPC is doing its part to further the goals of modern Communists and Socialists who have found a voice in the Democratic Party.