Currency manipulation is meant to benefit the country doing manipulation (in this case Switzerland) at the expense of another country (in this case U.S.)
It's very much the role of U.S. government to protect U.S. from other countries trying to do harm to U.S., be it by bombing U.S. territory, tariffs on U.S. goods or currency manipulation that economically hurts U.S.
You could present an argument that Switzerland wasn't trying to harm U.S. economically via currency manipulation but instead you're trying to delegitimize the very idea that U.S. can defend itself from other countries trying to harm it economically by pretending that it's purely internal affair that has no effect on U.S.
Currency manipulation does hurt U.S. and that's the reason U.S. has the right to push back on it.