He couldn’t. All he could do would be to dictate the code of conduct for physicists working at the MPI. And if you don’t like his code of conduct, you can go to a different institute, or take it up with the community or the board of directors for the institute. Your implied scenario of “what if a nazi wrote a code of conduct” doesn’t scare me because the only thing worse than a nazi is a nazi in hiding. And if the code of conduct contains no nazi agenda, well then he can be held accountable if he does something against the code of conduct (like, for example, being a nazi)