I wasn't able to see how the AI did it since I'm playing SE Asia nation. But from the map, it seems that Golden Horde managed to get to the sea very early from Crimea, and they since got colonizing ideas. They also beat Kazan and Nogai to reach Siberia before the Russians did, and colonized from there... These colonies are all theirs, not seized from Moscovy. And they are colonizing.
Maybe I'll see them reforming government, then westernize and annex Russia in this game! LOL