In some cases evidence is scarce, but in the story told in this article, there would seem to be a set of messages recorded in Slack, so it wouldn't just be one person's word against another. If the person being harassed wanted to make a really strong case, they would have sent a Slack reply saying "Don't ever send me this kind of message again!" to have a record of it.

This also makes it scary for HR, since if they do nothing and the victim sues, the Slack messages could turn up during discovery.

(I'm also guessing that this kind of behavior suggests that the aggressor is not of higher rank. Someone with real power over the victim probably wouldn't just stalk them on Slack, and any manager would be acutely aware that you don't want to say things like that in writing.)