Nah, this is definitely one of those just-so stories that’s too cute to be true. Like it sounds like the person who came up with it started with the idea of using American cultural stuff to tell soldiers apart (which maybe happened in some form at some point) and then worked backwards to try and justify why it would be a common practice with a harsh penalty (German officers who spoke perfect english because they… actually were American… but didn’t follow baseball?)

Edit: It reminds me of my favorite definitely fake boomer story: That people used to call out speedtraps on the highway by pulling over and standing in a salute… because cops can compel you not to alert people of a speedtrap… but they can’t compel you to not salute… because that would violate the first amendment? Before the internet dudes used to just sit around telling each other stories like this.