It was not a secret. But remember that Hitler's ideology wasn't so much nationalist as it was racialist. In his mind, the noblest "race" was the Germanic peoples of central and northern Europe, and Germany was merely the "vehicle" through which this racial superiority was best exemplified and through which domination over supposed lesser people's would be achieved (Hitler saw Germany as the standard-bearer of the so-called "Aryans," essentially, a group which he defined as including such people as German-speaking Austrians.)

So according to National Socialist race theory, non-German German speakers in places like Austria were considered part of the greater German national family and on an equal tier with German citizens.