Nicholas Kazanas doesn't have any formal training in archaeology, history or historical linguistics and this essay is self-published by him, so in that sense it's not a particularly reliable source. Some quick googling tells me Kazanas has a history of rashly wading into the Aryan invasion debate.

That's no reason to dismiss his arguments out of hand, though.

The basic premise is an apt observation. The problems Kazanas identifies in the AIT theory is the major problem in Indo-European archaeology more generally, namely:

how was the aryanisation of such a large area possible?

The linguistic evidence shows that at some point in prehistory there was a mass language shift to IE languages. We think we know roughly when and where the IE languages came from (again, mostly from linguistic evidence). But there is no archaeological or genetic evidence for mass migration, invasions, etc that fit. So how does mass language shift happen without mass population or cultural shift? We simply don't know. It's a paradox that applies to the spread of Indo-European everywhere, it's just particularly well articulated in South Asia, where the question of whether Vedic culture was an "Aryan invasion" or "indigenous" has a lot of social and political weight.

I don't have the expertise in historical linguistics or philology to pick over the body of the essay. Basically Kazanas argues that the Rig Veda (an Ancient Vedic text) is substantially older than everyone else thinks and therefore the Indo-European languages originated in India and spread from there. As far as I know this flies in the face of mainstream historical linguistics and Sanskrit philology. But as an archaeologist my main thought is rather that it misses the point – the point that Kazanas himself introduced at the beginning of the paper. There is no archaeological or genetic evidence that explains what "vector" carried IE languages from their original homeland to the vast area they now occupy. This problem is no less acute if the homeland is in South Asia instead of the North Pontic, so Kazanas has torn down a hypothesis with shaky foundations and built a new one on sand. He makes absolutely no use of archaeological evidence, superficial and inexpert use of genetic evidence, and rests his entire argument on his (apparently highly idiosyncratic) views on Sanskrit philology. It's just another grand narrative of Indo-European prehistory cobbled together from selective scraps of etymology, and those are ten-a-penny – you can find equally (not) well supported arguments that IE originated everywhere from Scandinavia to Tibet.

I read up to the terrible "linguistics" section on page two and I don't have any real desire to read further. It's worth noting though that the only people who argue that the PIE homeland was in India or worse, that Indo-Aryan languages are not IE, are crazy Indian nationalists. In terms of reliability, the entire paper is written in a pretty amateurish tone, and the author does not give a scholarly affiliation, so unless there's good evidence to the contrary you can disregard it.

None of his linguistic arguments make sense to me and the entire article seems incredibly amateurish.

I think the burden with something like this should be on those who would give him any credit rather than those trying to discredit him. There is a reason this is self-published. Anyone can write anything and put it up on the internet.