"Scared of public channels" often means "doesn't have the psychological safety to say something out loud that might end up being perceived negatively or as though they don't know what they're doing."
Sure -- or people don't want to clog up channels with what they perceive to be minutiae or very context-specific questions requiring some back-and-forth.
Which is fair, as long as it's concrete, the addressee is actually available and on the clock, and the question can only be answered by that person.
I'm active on a programming language Slack space, what sometimes happens is that people are DMed directly with a generic / open question that dozens of others could also answer.
For which, I am thankful to them. Nothing better then dozens of irrelevant chat messages in larger group chats I am member of that I cant mute, because once in a while there is something important.
I know for a fact that anonymity can be used to encourage diversity of thought and honesty on public channels - this is the feedback I get from customers of a service I run (https://AnonymityBot.com).
I personally don’t have a problem with posting in public, but I can imagine that for many people it would remind them of being singled out in a school class.