IRRC whilst the Austro Hungarian empire was allied to the German Empire in 1900 they were two separate countries. Austro Hungary included Bosnia, Croatia, Slovakia, the Czeck republic and the Tyrol.. The German Empire was formed by Prussiai in what is now west Poland and the current Germany. They were at war in the 1860'a- Although Germany of 1900 isn't the same as todays Germany.
Germany in 1900 was part of the Austro Hungarian Empire, which consisted of The Kingdom Of Prussia, and The German Empire. These consisted of many places which are not part of modern day Germany, so I'm not really sure why the Germans are that arsed about this history - considering it was "German colonisers", these could have came from any number of countries and be nothing to do with "Todays" Germany.
ETA Late to the party as usual