Why was he considered to be Jewish?
He would be considered Jewish by Jewish religious law, AFAIK, which would be an OK association for one who accepts Jewish religious law.
Also, Jew-haters identify one as Jewish (or at least in on the Jewish conspiracy) on a basis as remote as one Jewish grandparent or even less. Sometimes just a ‘Jewish sounding’ last name will get one a Jew tag (the accused presumed to be covering up his Jewish affiliation).