If I’m out of office they can still type the message and I’ll see it later. If they can’t wait and would send it to someone else if I don’t immediately respond to the “hi”, then they could either copy-paste it, or - even better - send it to some relevant channel that contains multiple people capable of assisting them instead of PMing it to me.

As long as it’s just an “hi” you don’t know what it is and have to assume it’s super urgent. Once you answer, they can disclose the actual matter in its full mundanity, resting assured they have already confirmed your attention.