Ok so GB and the UK are both dominated by England. It wasn't the Northern Irish forcing Scotland, Wales, and England into a union. The Scottish aren't the dominant culture. The Welsh aren't responsible for most of the cultural output and historical exploits of the British Empire. A lot of people, not just Trump and not just Americans, often see England as being, in practical terms, what's particular to and about GB/UK.

The vast majority of people in GB and the UK are English. The capitol is in England. It's the cultural, economic, and political center.

Also just using the term "England" in reference GB/UK doesn't indicate that someone doesn't know the specifics. It's a colloquialism.

Brits almost always call people from the US "Americans". If someone says "Americans are cunts" they're not referring to Canadians or Chileans.

Trump is guilty of using an unspecific colloquialism. British people do it all the time in the other direction.