The Mongols were a mix of modern and violent and ruthless before he (Chinggis Khan) died that you don't really see elsewhere.
20 years ago-ish, a book came out about Chingis Khan that extolled the virtues of his multicultural, pantheistic regime and totally glossed over the many genocides the Mongols perpetrated. Not to mention the guy probably raped more women than anyone else in history.
Edit: To add to this, a lot of what is written about Mongols in Chingis Khan's time is conjecture. They were not a literate people, so there are almost no records. They were nomadic and buried their leaders in secret locations, leaving scant archeological evidence. Many conclusions that have been drawn about the Mongols are speculative. Fast forward to the Yuan dynasty where there are records in Chinese. That era and locality have better documentation.
Most of those things you said can apply to the Assyrians, the Romans, the British, or any other similar empire, and people still idealize those places today. If your empire holds together long enough to get a period of Pax named after you, it means you've made it in the eyes of history.
I didn't see any wholistic judgment of the Mongol empire in the article.
The Mongols did tons of amazing, fascinating, and great things. They also did terrible and atrocious things.
The later should not preclude curiosity and appreciation of the former.
People tend to ignore their ancestor's warcrimes. Even if they admit they were evil they can't bring themselves to hate them.
This applies to most old empires, many people can't bring themselves to hate the entire empire despite their barbarity.