Maybe the classical era too, I don’t know where the start year should be. It ends in the early modern period when bordering agriculturalists like the Russians start expanding.
In other places and times agriculturalists tend to displace nomads on arable land, probably because crop farming can support a lot more people (and therefore fighters) per area.
Any explanation needs to be valid across the whole period and rely on things the nomads had that the farmers didn’t. Horse archery was not new by this period.
It’s not just the Mongols, though. The Golden Horde was a thing centuries later, and even when the nomads were too busy with infighting to expand into Europe, China or Persia they kept the farmers out of their home steppe.
I’ll watch John Green, but I’m pretty skeptical of the whole bow theory for that and other reasons.