I might be wrong, but I did not perceive this as him threatening you in any way. In true German fashion, he did what his attorney told him to, asking you to preserve the data of these incidents, surely for legal action against the mentioned user. Doing this requires him to put his real name there otherwise ist won't be accepted. Is this a questionable ask towards an uncensored protocol that gives no guarantees for data persistence? Hell yes. Telling him to fuck off if you don't want to get involved is perfectly fine, imo. But you do not doy people with their real names, ever. Just my 2 Sats ofc.