Correct me if I'm wrong, but I think the person on the call wasn't Huffman himself. Selig was talking to some Reddit employee, who presumably then told Huffman about the call.
It's possible that the employee who talked to Selig didn't 100% accurately describe what was said in the call (because he didn't have a transcript to check what was said), and then Huffman misunderstood whatever the employee said further to come up with the blackmail accusation.