English-speakers used to use it to mean all non-Scandinavian Germanic peoples. When the Netherlanders became a distinctly separate group Britain had way more contact with them than with anyone else that the word used to cover, so we used it to refer to them specifically
If I remember correctly, this is also one of the leading explanations why the Pennsylvania Dutch are called like that even though they speak German (or a German dialect).
Germans in America were a massively more influential subculture before WWI. Notably: not because of WWI. The heart of German-American culture was in New York City, where all the richest families took a boat for a holiday cruise, and one year it sank.