In the context of this thread, Mitanni means the ruling class obviously. Its a bit cumbersome to write ruling class of the Mitanni all the time. That said, they "certainly" must have been Indo-Aryan, Sanskrit Speakers from the Indus or North India? You can be certain about the first bit, that they were Indo-Aryans. The rest is pure conjecture, and not even technically supported by the Linguistic evidence, since as pointed out, their language is more conservative than Sanskrit, and doesn't indicate an origin from Sanskrit, rather a divergent evolution from a common ancestor. Where was this common ancestor when this split happened? That's impossible to determine with the evidence currently available.
The language was similar. It looks more conservative, but it isn't. It's just the translation. Were they written in Indian script and then translated no doubt they would be the same.