America will prosecute Americans for doing certain things that are illegal inside America outside its borders. As another example, if you take a boat to international waters and kill someone on it, you're going to get arrested and prosecuted when you get home.
America will not arrest or prosecute someone from the UK visiting Thailand as a sex tourist.
In this case, the operator of 4Chan is free to blow off the UK's law. They may wish to account for that in future travel plans, though.
But given the behavior of the UK government lately, doing something suicidally stupid seems on brand for them.
Every time the media reported something like this, turned out they were leaving out something important. Like the professor who was smuggling biological samples into the US. Turns out that's illegal, that's why she went home. If you aren't doing something like that, you will be fine.
However, if I'm going to break one of their laws that they feel very strongly about, I'm probably not going to travel to the UK. That's just begging for something bad to happen. Why risk it?
So in this case, if you know the US is looking for you, why, oh why, would you travel to the US?