Hi Franky,
That is a great question! Basically, the zone of indifference is not an option in the Generate Spatial Weights Matrix tool because we made the decision that for almost all use cases the difference between zone of indifference and fixed distance band is very, very small. Once the distance band (d) gets large (even above 10, for instance), the weight of features outside of that distance band is 1/d...which becomes negligible for most use cases). We still have zone of indifference in the Hot Spot Analysis tool and others because once you provide a parameter option you have to continue to provide those parameters for backward compatibility, but when we designed the Generate Spatial Weights Matrix tool we decided not to include it. I would suggest running the same analysis with Fixed Distance Band, and you should see very, very similar results.
Hope this helps!
Lauren Rosenshein
Geoprocessing Product Engineer