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.

A population which to a large degree is often found clustered in separate communities (as the case for many minorities in other countries) and generally segregated, even if not by intention. Coming across a Muslim can be an oddity in many localities. There are a lot of communal tensions still being harbored by people today. I have repeatedly heard the phrase "सब पाकिस्तान से हैं" (they are all from Pakistan) being used derogatively to describe Indian Muslims, amongst other similar things (that one sticks out to me because I've heard it way too often)

It is possible that the reasons are not what the author thought they were. But it is pretty likely that they were, and I'm suggesting we give the author the benefit of the doubt in that he had valid reasons and prior similar experiences to feel that way.

It is very easy to discount an experience when your bubble makes you think it is impossible. For many years I didn't think casteism was a thing anymore in cities, since I never saw it happening. Over time I realized that it is still very much there, just that those not negatively affected by it tend to not notice it.