Well mister, I really don't need to be lectured on the iron-carbon equilibrium diagram, but I don't mind taking a closer look at the temperatures mentioned.
"By comparison, steel becomes malleable at about 2,100 degrees."
The only identifiable phase change "at about 2100 degrees" that occurs on the equilibrium diagram is the solidus that occurs at 2066°F when Ledeburite freezes for alloys containing more than 2% carbon (generally referred to as "Cast Irons")
The term "steel" is generally applied to Iron alloys containing less than 2% carbon, however the "soft" steel (0.1%) that you refer to, or "mild" steel (0.2% to 0.3%) contain significantly less carbon. Yes, these low carbon steels have melting points somewhere around 2700-2800°F if you ignore distinctions between the solidus and liquidus phase changes. But there is no phase change at 2100° in this region of the chart where steel would magicly become "malleable".
In fact, "hot working" of steel generally refers to metal forming that occurs above 1333°F -- although forming is easier (and preferred) at higher temperatures that are still below the solidus. Nevertheless, steel can still be "cold" worked at lower temperatures to the extent that it is still "malleable". Granted, it may be somewhat less "malleable" than at higher temperatures, but there is no critical 2100°F temperature where it suddenly "becomes malleable" as this article asserts.