Anyway, from the Wiki page. Walsh uses the German loanword, not the German word, which means his spelling is correct. It modifies a noun so it should not be capitalised.
Ersatz is a German word[1] literally meaning substitute or replacement. Although it is used as an adjective in English, Ersatz can function in German as a noun on its own, or as a part in compound nouns such as Ersatzteile (spare parts) or Ersatzspieler (substitute player). While the English term often implies that the substitution is of unsatisfactory or inferior quality, this connotation does not necessarily exist in the German context. For example, "Ersatzbutter" or "Butter-Ersatz" could be used as a generic term for margarine as a substitute for butter.