That certainly looks reasonable to me. It’s not obvious, but if you add ‘print’ statements to your filters, you can see them in the Output window.
In my filter, I was unable to use vtkDataSet.GetData(outInfo), I had to use a more specific type like vtkPolyData, but the example clearly uses vtkDataSet, and I’m not using numpy.
I suggest adding print(output) to see if it’s doing what you want.
HTH,
Aron