This heuristic is commonly believed — and expressed on this website — but is idiotic. Plenty of people have known how to use em dashes when writing on a computer since the 1980s. (This was true even before UTF-8 was invented to make extended characters work across platforms; instead of ⌘⇧-hyphen, we'd just type "--" to approximate it.)

The reason LLMs tend to use a lot of em dashes is that the human-authored texts they were trained on used a lot of em dashes.