IANAL, but it seems to me like the US could totally pass laws regulating Germans on German soil. It just couldn't enforce them in US courts (short of kidnapping Germans from German soil, or luring them to the US somehow). So it would be mostly pointless but not impossible.

Also, multiple countries have laws claiming universal jurisdiction. As I understand it, French laws against genocide denial claim to apply universally.

From your own discussion, it seems like "subject to the jurisdiction" should be understood as "can be judged in court". Diplomat is immune = can't be judged = not subject to jurisdiction. Immunity lifted = can be judged = subject to jurisdiction.