It really is though. In most cases all they can do is kick you out. In some cases the foreign government can waive immunity to allow prosecution, but it’s rare. This guy wasn’t a diplomat though so diplomatic immunity doesn’t apply. Regardless of that, my understanding from having grown up going to an international school in NYC where most of my friends were there because their parents were assigned to the UN, is that there’s a weaker form of effective immunity that basically is just that the UN grounds are not trivially part of the US. So it’s not clear to me why US bribery laws applied to a foreign national on foreign business inside the UN? My guess is that the practical answer is just that the US are world police and so they do what they want.

https://en.m.wikipedia.org/wiki/Diplomatic_immunity