One piece of advice I got from my manager early in my career that really stuck with me is this:

- It’s okay not to respond to emails/messages/requests immediately.

- But if you know a response will take time, acknowledge the message and say so.

It’s simple: the sender gets a clear signal that their message was received and isn’t left wondering whether it got lost, accidentally deleted or ignored.

This doesn’t actually contradict the Napoleon technique... if anything, it softens the “he's ignoring me” factor while still protecting my time and attention.