"No stall speed" just means no stall break in the traditional sense. The AN-2's slatted wing's maximum AOA is very high (typically about 10+ degrees higher than un-slatted, or mid-high 20s vs mid teens) and there isn't enough tail power to get the AOA high enough to get any kind of break.
Stall speed is normally published as the speed at which the wing is at maximum lift coefficient. A typical wing makes a Clmax of about 1.5, slats raise that to about 2.5, and a slotted flap raises that again to about 3.5-ish. The AN-2 has full span slats and flap-flaperons so you can assume that just about all of its 770 sqf of wing area is producing a Clmax of mid 3s with slats out and flaps down.
You can use the handy calculator on this page (click on the "Stall Speed Calculator" link) to work out what the speed is at which this is achieved. For the AN-2 you could assume with 770 sqf of wing area, with, for argument's sake, a Clmax of 3.5 for the entire wing, and operating at, say, a relatively light 9000lbs, you get a "stall" speed of 32.8 kts (37 mph), the speed that the wing is making maximum lift based on those assumptions and technically, this is the answer to your question.
With a slatted wing, the lift curve has a more rounded top; this is, lift declines fairly gently after the peak, so you can continue raising AOA and all that happens is sink rate goes up, until the tail runs out of downforce authority.
With power, thanks to the slats, you get both more tail downforce with the slipstream blowing on it, and because you can reach very high deck angles, you start to get a significant boost from the vertical component of engine thrust itself when the thrust line is canted up at, say 25 degrees. And on top of that you get added lift from the slipstream itself passing the wings..
If the engine is making, for argument's sake, 4000 lbs of thrust when at full power (you get roughly 4 lbs/hp) and the thrust line is canted up 25 degrees, there is a vertical thrust component of about 1700 lbs. This has the effect of reducing your weight by 1700 lbs, and plugging in a 1700 lb weight reduction into the calculator gets you down to about 29 kts (or 33 mph). Add in the additional lift from slipstream passing part of the wings dropping the minimum speed a few more mph, and you can see how you can get to the point of the plane being able to back up at 5 mph in a 35 mph headwind using power in the slow-flight regime, at a relatively light weight at least.
So, the AN-2, at a relatively light 9000 lbs, using the standard formula and using typical assumptions, should be able to fly at a minimum speed of 37 mph power off and about 30 mph or a little less power on without sinking, which agrees more or less with the article's statements.
Plus, there is enough tail power with power on to increase AOA a little more, getting speed down to 25 mph, but while it's under control, total lift will be declining and the airplane will be sinking while doing so.
None of this is particularly special. The AN-2 achieves its performance with lots of wing area and full span slats and flaperons. Other STOL aircraft with full span flaps and slats can do the same thing.