No. For hundreds of years authors loved to use the em dash. It was a sign of quality writing. Only very recently has it stopped getting used and only because the - is on the wrong side of the keyboard and it's becoming more a hyphen instead.
When someone uses an em dash, it implies they arent using a normal keyboard; not even dvorak.
Even today it's common in professional writing (high quality articles, published books). I agree though that a typical modern keyboarder is going to use a dash or semicolon instead (if anything).
So maybe the training data has a lot of old English writing and overcoming the model's tendency to use em dashes everywhere with custom instructions would use up more electricity.
When someone uses an em dash, it implies they arent using a normal keyboard; not even dvorak.