Hehe man check the area they're talking about:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2253976/figure/fig2/
I'm sure they are correct about THIS SPECIFIC area.
This is where the original Russian Slavs settled before Mongols came.
Part of this area even was free from Mongols (Novgorod land, for instance: Mongols never touched it).
Now imagine guys like Shlemenko: born and raised in Omsk, dunno how many generations of his ancestors been there.
Omsk was founded in the middle of 18th century, and was 100% Russian town, but they had a lot of Siberians and Mongols to mix with.