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.
The comment you responded to used "imperial", which is the adjective derived from empire.
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.
Imperialism does but not the adjective 'imperial' which simply implies relation to an empire. Imperial Japan has nothing to do with the British and yet it is 'imperial'.
Yes you are right about the phrase "Imperial Japan" not having any relation to the British. But "Imperial India" does.
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?
I understand what you are saying but you've asserted as truth that the author said what you said and in a pretty accusatory tone. If you use 'Imperial India' as standalone phrase it does refer to the British rule because it is the most recent and that's what most assume you mean. But when talking about Buddha it is quite a leap to think the author would use such a pointless timeframe so the context alone is enough.
Even a paragraph later the emperor Asoka is mentioned explicitly
