Group by parent terms from indexed node in views
You can also add the parent term to the index.

So if you have field_category as your term field you will be able to index field_product_category:parent.

Note that you may have to use the "Add related fields" section at the bottom of the fields admin page for your index to add the parent term relation before you can add the parent term tid or term name etc.

Then in your view also add the parent term name field and exclude it from display and then group by it.