The topic of automation is a whole big conversation

Of course. With a lot of components to consider. I have already read many posts or comments from you regarding the automation of various processes on the Steem. Our opinions often diverge somewhat, but never completely. I don't demonise automation at all (especially since it simplifies a lot). But it has to be constantly maintained by real people.
E.g. the former plagiarism bot @cheetah. I liked him a lot... ;-) But at some point he started to strike as soon as he discovered a repetitive footnote. When I pointed this out, its operators did not remove the automatic downvotes. Worse still, the users ended up on a blacklist in case of repetition and had no chance of rehabilitation because the people responsible no longer cared about the bot.
I know @cheetah doesn't really fit the topic now, I just wanted to get away from the vote bot.... ;-)

mod-bot isn't actually voting for any posts.

Yes, I understood that. By pinning the articles (and thus making them important), he influences other voters indirectly at best (as your son already said). That's what promotion is for, advertising.
And here, I think, the selection has to be careful. Go beyond parameters and bring human considerations into play. But I'm certainly not accusing you/your son of not doing the latter!

In the end, the moderator will have ultimate (manual) control over both pinning and muting.

That should be out of the question anyway. But for that, the mod has to control the articles. In that case, he could just pin them by hand... ;-)

This mod bot is going to be great and I think it's super cool that you're programming something like this! Try it out, evaluate it, declare it perfect or improve it. The best prerequisites... :-))
I reserve the right to decide against its use in my community at the moment - that's what @cmp2020 asked about.