area North of Mongolia was predominantly pagan\shamanist at the time of the game, with the growth of Buddhist influence during Qing dynasty's rule over Tyva and Mongolia.
Islam was introduced to the area only during the later years of Russian Empire & Soviet Era, during worker migrations, that's why there are some Muslim villages today around Novosibirsk & Krasnoyarsk city areas. There are Sunni Muslims also in south-east area of Altay republic (Kosagash or Kosh-agachskiy rayon) where ethnic Kazakhs live.
But this is all a result mostly XX c. events.
That's why presence of Islam in Buryatia and other areas to the north of Mongolia is gravely inaccurate, and should be addressed by the developers