There is one nice scene when Elizabeth is having dinner with Porchy when they allowed Olivia Colman to use facial gestures.
My gut reaction is that he was talking about Montgomery. I had uncles who fought for the United States who made similar comments about Monty. There are two famous war movies, Patton and A Bridge Too Far, wherein Montgomery comes across quite badly. Monty doesn't appear in the second movie, except in some brief archival footage, but Americans who follow war history generally give Monty the lion's share of the blame for that debacle. I am not aware of any stories that blame Mountbatten for the loss of American lives.