Saying “hi” back implies that I have time to listen to you right now. Maybe I don’t, or maybe I’d like to wait to decide until I know what you want.
And it’s not always “hi” anyway. I used to have one of our C-levels DM me on Slack almost daily with some form of “hellllp!” No context whatsoever. Eventually I figured out they had a list of people they thought were helpful and would just spam that list for any kind of thing they needed. I stopped responding and got myself off their list. Problem solved haha.
While the original article doesn't mention it, I think there are two issues here, one of which you allude to.
1. Someone in a position of authority making context-less contact: "hi", "helllp!"... this is going to cause anxiety and fear in nearly anyone and leadership should avoid doing this at all costs.
2. Regular people communicating in a "normal" way by starting a conversation off with "hi". Maybe they're not so online as to be honed with IM etiquette, maybe they just like to say hi. Regardless, their intent is not malicious and a healthy and collaborative culture should be able to handle their natural (and quite popular) social communication style.
And it’s not always “hi” anyway. I used to have one of our C-levels DM me on Slack almost daily with some form of “hellllp!” No context whatsoever. Eventually I figured out they had a list of people they thought were helpful and would just spam that list for any kind of thing they needed. I stopped responding and got myself off their list. Problem solved haha.