What’s the categorical difference between the nodejs repo and the max planck institute that it’s ok for one to have a code of conduct but not the other?

One is a piece of software, governed (one hopes) by the terms of its license if at all, and the other is a group of people, or (according to some people's beliefs) some sort of incorporeal tulpa-like entity whose activities are carried out in the material plane by a group of people.

I don't know why you keep asking me questions that are answered in the document I linked at the top of the thread. It's less than 1000 words long, you can read it in three minutes.

Because your document doesn’t answer any of these questions, and I’m confused why you think it does.

You don’t actually specify any beef with what you call purpose #1 - an agreement within a group about how it should be governed. Except for, as you say, single developer or small group projects - that’s the stated purpose of ALL codes of conduct. Some may contain ethical postering or demands of users (i’ve never seen that) - but those are malformed codes of conduct and should be criticized for those flaws not for being codes of conduct