Mongols didn't just 'withdraw' and leave Europe alone. The article mentions they brought Chinese gunpowder - so they left behind a valuable technological legacy.
They killed one million Hungarians, but that was just one nation. The overall conquest(s) effected a vast death toll across Europe and near Asia.
What else did they leave behind? Not mentioned: a genetic legacy. This was a living legacy as impactful as anything else, arguable even more so...
And what's involved in that? Conquering armies tend to kill men and boys, and rape women and girls. (The most comprehensive recent record of similar events is from post-war Germany: https://en.wikipedia.org/wiki/Rape_during_the_occupation_of_...) A significant proportion of those rapes resulted in pregnancies. In medieval Europe, we can assume that abortion was mostly unavailable.
So actually, in one sense Mongols remained in Europe, spreading their genes through the most vile and traumatic acts inflicted on the hidden victims of conflict: women.
Women are half of humanity. It's wrong to neglect that aspect of invasion.