That's just plain bullshit and you know it. That sort of talk began back in the McCarthy era.
The ACLU will defend the right to free speech, even if a person happens to be a communist. It doesn't make the ACLU a communist organization.
From the ACLU website:
"The ACLU believes in the freedom of thought, inquiry or advocacy by any individual or group regarding a specific economic, social or political philosophy. That position, grounded solely on the nation's tradition of free speech, means no governmental restriction unless there exists a clear and present danger of illegal action."