This article is attacking a strawman. Nobody was ever advocating for labeling all em dash usage as AI. Even the tweet they reference (not that random tweets ought to be taken as some sort of authoritative gauge of the current state of society...) does not claim that all em dash usage is AI.

In certain contexts, em dashes are perfectly natural and human. That being said, everyone has encountered articles and posts that read so obviously like AI, and in those contexts the presence of numerous em dashes is certainly an additional data point.