I like that you ignore that “in” means “inside” in one sentence and then use that exact meaning of the word in the following sentence

The term EU describes a membership, it doesn't describe a geographic area. Being in a membership means something different than being in a geographic area. So the term "in" does not have the same meaning across the two contexts.

The term EU doesn't describe a geographic area. It is a membership. Just as if I'm surrounded by members of a book club, I'm not in it.

By that light, the UK is still in the EU. I bet they would disagree though.

The UK is not enclosed by the EU.

To be pedantic: neither is Switzerland because of Liechtenstein...