Criticising past immigration is something different than inciting an audience to chant to reduce the number of Dutch citizens with a specific background.

Most political parties in The Netherlands have talked about the problems of immigration and integration in The Netherlands at least since Pim Fortuyn 15 years ago. Some parties longer (e.g. Frits Bolkenstein of the VVD, who Geert Wilders worked for). So, it is by no means illegal to talk about current or past immigration policies.