I think he is saying that once US citizens return to the US, then they will be arrested.

The US has seized non-US citizens, abroad, for acts committed abroad, over which the US asserts (and exerts) extraterritorial jurisdiction, not just US citizens, and not just waiting until they enter the US on their own.

If they were talking about the US arresting US citizens, then the equivalent would be Ofcom sending a fine to the UK visitors of 4chan. That's clearly not what they're doing.