Everyone Is “English”
It doesn’t matter who you are or what your background is — any person outside of the Amish community is technically considered English. This is because when the Amish first came to America everyone they ran into were English settlers.
The Amish are well known for traditionalism and really refusing to change over time so this is just another one of their customs they decided to keep. They couldn’t call people Americans because the Amish technically consider themselves Americans too.