The horizontal tail can't make sufficient down force to slow the plane to get the slatted wings to stalling AOA. With the slat system, the stalling AOA will be perhaps 10 degrees higher than without. With the AN2's large wing area, to get the slatted wings to stalling AOA might require the speed to drop to, say, 20 kt. But the tail simply runs out of down-...
As you said, the Y-5 and Y-5B are Chinese variants of the Antonov AN-2. The Wikipedia page gives this info on the Y-5B:
Shijiazhuang Y-5B-100 – Y-5B aircraft fitted with triple tipsails on the upper wing tips, which reputedly gave 20% higher climb rate and improved L/D ratio by 15%.
Here is a research paper on tipsails which may help a little bit.
This site states it a bit better:
Stall Speed: 35-40kts. but controlled descents are possible at 25kts. or 30mph.
In reality most aircraft have the ability to glide at some speed. The AN-2 simply has the combination of a quite a good glide ratio and a low stall speed so the aircraft can be settled to the ground in a controlled manner.
"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 ...