You are right where pivotGrid works with ExtJS since 4.1. The problem is that this is like an extension to ExtJS. It is not (at least up to JS 5.x) included in the main package and must be bought separately.
As the license Ext.NET provides to ExtJS is limited to the main product, we do not include the pivotGrid extension itself and, as we don't have that extension, we couldn't build Ext.NET support for it, unfortunately.
If it becomes part (native) of ExtJS 6 (future Ext.NET 4), then we will most likely be implementing support for it on the upcoming Ext.NET v4 release.
Hope you understand.