Let's see if I understand what you're saying...
"A communist (Mamdani, I assume?) saying he supports something about a non communist (FDR, I assume?) does not make the non communist (FDR) a communist. No offense, but I think that’s a straw man."
If I'm reading you correctly, well, I agree that Mamdani saying he supports FDR policies doesn't make FDR a communist.
But, it doesn't make Mamdani a communist either. He says he is a democratic socialist, which is not communist. While both philosophies may reside on the same side of the political spectrum, words have meaning and they are two different things. However, to a person whose own politics are far enough to the right, the differences between democratic socialism and communism may be of little consequence.