That’s literally an argument against anything that exists at all. That’s kind of how laws work, linguistics is complicated so everyone’s interpretation is different. But as it stands, communist parties are not banned. What you speak of is a big “what if”, and currently you saying communism as a whole is banned is simply wrong, even as an oversimplification.

It is a big stretch to turn “Parties other than the two largest ones in the country have considerable cultural, legal, and logistical obstacles to being able to participate in high-level American politics, and an unenforced law from 70 years ago banned one specific communist party before most of the provisions being repealed by congress and thr law being overturned in state courts as unconstitutional” into “Communism is banned in the United States”. There is no legal way to criminally prosecute someone on the basis of them being a communist in the modern day.