Why are you assuming that they are referring to the British Empire? They could be referring to the https://en.wikipedia.org/wiki/Maurya_Empire, which covered most of India not long after Buddha is said to have lived.
Edit: Imperialism means to extend over "foreign nations". Mauryan empire is not foreign, it is as Indian as it can get.
There's no other adjective for empire, and there's really no reason to assume that use refers to modern meanings of imperialism (which is itself the noun derived from imperial...), meanings which are themselves derived from specific behaviours of recent empires.
Any reader searching this phrase online or looking up any common reference will find the British relation. Unfortunate, but that is how it is. As an Indian, I have always known that. That is why the phrase jumped at me.
Is it too much to expect an author writing such a detailed article from this region to know that?
Even a paragraph later the emperor Asoka is mentioned explicitly
The Mauryans built their empire by extending over the politically and linguistically disparate nations comprising South Asia.
I would be happy if you could. If not you proved my exact point again.