Fact is,it was at that time the german city of Posen.
That's debatable. Why not Polish city inder German administration. In fact, Poznan is one of the earliest Polish cities and Poles were always a majority, except for about a decade long period in the middle 0f the 19th century. Even during German occupation, Poles living referred t the city by their original and present name.
A frenchman in London may call it Londres but its still London.
There is a significant difference. A Frenchman in London is a visitor/newcomer. Poles living in Poznan had been living there for generations.