The article is here.
Follow up question: Assuming that it isn't true, can we see anything like a skill progression or accumulation of knowledge when it came to steppe peoples' ability to take a city?
The history section of that article is one of the worst I've seen; it barely follows wiki conventions.
Is it accurate? Eh. I can think of no reason a historical article could say "These people couldn't do something." Such a statement is entirely invalidated by an "except" statement, and is then simply hyperbolic. It would be better to state the people of the steppes were inexperienced in seigecraft and had developed military tactics that worked in less densely populated environments. A nomadic group fighting other nomadic groups has better things to think about than taking a fortified urban center. Their military was indeed focused on cavalry. Horses excel at running with an armed rider, intimidating enemies, and being several times larger than a person; they are not good at climbing ladders, operating siege equipment, or siege battles of endurance and out waiting.
In addition, the Khans were far from the only ones successful at invading neighbors. Steppes-folk established multiple dynasties in China, as mentioned in the article, and this did require taking defended cities. The first to do so, the Xianbei, held northern China for 200 years during the Six Dynasties period, an impressive feat for the turbulent age. They did began to lose power as their military rule was insufficient to handle civil affairs, but this was (mostly) solved by incorporating locals into positions. So could they take walled cities? Yes. Were they the best, or even exceptional at it? Not really.
Tang, Chagru and Dein, Albert in State and Society in Early Medieval China
It's a ridiculously broad statement about a very long period of human history characterized by vast geographic variation and technological change. It's also not true. The Huns took walled cities; so did the Manchus and Junghars, as well as (I think; not 100% sure of this) the Jurchens and Khitans. Traditionally the limitation on steppe empires was extending their power base outside of regions where grazing lands were available, as well as dealing with succession crises and other forms of political fragmentation in the absence of a continuous bureaucracy not belonging to a conquered population (or else resisting assimilation into that population).
As for skill progression, someone else might be able to comment on this better, but generally speaking this depended strongly on whether the nomadic empires found ways to appropriate the technological advances of agrarian societies, which of course changed radically depending on the period. Probably the most famous example of this is Genghis Khan and his Chinese engineers, but it's also possible to cite the example of the Junghars, who had a captured Russian/Swedish engineer create an artillery corps for them, and the Manchus, who used the Jesuits for the same purpose.
If one considers Tamerlane to be a "steppe person", then no. He invaded Georgia several times, and on his first invasion in 1386-87:
"The capital city [Tbilisi] withstood six months siege, Bagrat [V, king of Georgia] coming out to counterattack several times a day. Timur Lang had to catapult rocks at Tbilisi's wall before it fell. Bagrat fought on the battlements until 22 November 1386, when Timur fired the first cannon seen in the Caucasus."^1 There is other mentions in the book of conquering castles throughout Georgia, but that is the only one to explicitly mention a siege of a city.