Regardless of what the FAA says, Delta should be allowed to implement their own ban on travel for this couple. If for no other reason to protect the safety of their employees.
It is interesting how the woman was fined, and not the male companion. I suppose it comes down to witness statements and what evidence the FAA has to collaborate the complaint. It seems obvious that numerus other passengers potentially witnessed the attack on the flight attendant when they returned to the gate, but I suppose no one came forward to witness his refusal to wear a mask, fasten his seatbelt, or put up his tray table. All of those are a violation of CFRs too, and subject to a fine.