DWD is quite unrestricted. Data is also processed, mixed with different models and many weather variables are calculated afterwards using other models. It is not showing raw data directly.

As far as I interpret terms correctly, I have to indicate that I am using open-data from DWD, but for value added services, this attribution is not required for all applications using it. I will do some additional research in this regard, whether applications using Open-Meteo should also display a disclaimer.