Surely all this talk about "not using a base" or "curry from scratch" is simply a traditional Indian method of cooking curries?

The difference being that:
a) BIRs generally make a large pot of a mildly spiced and, therefore, versatile, base that can be used to make a multitude of curries, for time-saving convenience
b) A small-scale version of the base is basically the same as the large-scale version except that the convenience factor is lost for making multiple curries
c) A traditional method is essentially the same as a small-scale version (i.e. an onion/garlic, etc, base made "from scratch") apart from the fact that you might choose to blend it to get a smoother consistency