I suspect he has experienced it often enough to know what is going on, he just didn't provide the full facts of why he thought so, which is understandable since that's not the subject of the article.

I've been living in India for nearly a decade now. Casteism and religion-ism are very non-obvious to those unaffected (or positivrly affected) by it, but spend enough time outside your bubble and you start realizing that it exists often in an undercurrent and start recognizing situations like the one he talks of for what they really are. I've seen far more overt versions of his anecdote play out; I have no trouble believing him there.

India has the second largest Muslim population in the world; it's unlikely that whatever tone he inferred was from being Muslim. More likely is the case that the passport control person saw the British passport and commented on that.

Moreover to state that the immigration officer inferred him speaking English as a sign of class is in itself racist; English is one of the official languages of the country, plenty of Indians speak very fluent English.

Having lived in the deep south, I found racism very obvious to those affected and unaffected by it.

Never seen anything like that in India which moderates societal differences to a very high degree.