I didn't know this.
I didn't know this.
It's not a proven fact, just a conclusion somebody jumped to.
FMD did follow the Communist Party line in his journalism and presumably was a party member, but not everyone in the party was a spy who reported to Soviet handlers. Those whom the FBI called subversives were more likely to be agents of influence who spread communist ideas and promoted the party lines as actual spies.
There were 80,000 members in the CPUSA at its high point in 1944. The USSR didn't need that many spies and wanted them to be as professional as possible. If every spy were in the party and every party member were a spy it would have been easy to break the spy rings.