Technically, yes, he was. Although, as Nielsio points out (which you probably saw), by strict Austrians he was not a ‘main line’ adherent of the school. But for generally all intents and purposes, he is considered a main, if not integral, part of it. He was a prime student of Mises, and much of his work is based in Misesian fundamentals. (Not to mention he actually was literally Austrian by nationality). His bio from the Mises Institute can be seen here.