This question is similer to my previous question.
I came across a person who makes the following claims:
The names of Previous Kings of Mitannis that are mentioned on inscriptions belongs to period of Late vedic era we don't find Names and such words used in Rigveda and Its a Undisputed fact that whole rigveda was Composed in Sapta Sindhu or Panjab-Doab Region. Plus the Numbers and Color names that were found on the Inscriptions are also Late Indo Aryan Prakriti words and cannot be Prior to Rigvedic terms and words. Plus it don't even have Iranian Feature In Which Sa changes to Ha For e.g the term Sapta is Same as Prakriti and Pali "Satta" and not Iranian Hapta Or Hafta. Also they Celebrated seasonal Solistice as their main festival and Term for that is also Belongs to Prakirt Late Indo Aryan voccabulary......(Vishuva) (still found pahari languages of North West as Vishu/Visova or Basoa or also called Baisakhi). How ever these features are absent in Early Vedic Times Indicating the mitanni ancestors were immigrants from Indus valley-(1700B.C) or post Vedic times.
Now, as we know, the Indo-Aryan loanwords in Mitanni inscriptions are said not to be derived from Vedic Sanskrit, but from Proto-Indo-Aryan. But how do linguists determine this?