There certainly is such a thing as stalling speed SSD, it's just that unlike AoA, it's dependent upon weight, configuration, deceleration rate and g. Stalling AoA is only dependent upon configuration and deceleration rate.
Draggy / slippery is really not the issue here. The aircraft I know with Vref>>1.3Vs tend to have high induced drag, low inertia. In other words, as you roundout and flare, there's a lot of drag acting against not very much momentum. The result is rapid deceleration.
A motorglider is slippery - yes. But also they're heavier than most - probably all - microlights, and with that long slender wing, have very low induced drag. So, the speed bleeds off very gradually, and thus a relatively low approach speed can be appropriate. By comparison, most microlights have low mass, already low approach speeds (remembering that kinetic energy goes with the square of speed, so reducing approach speed to 80% reduces kinetic energy to 64%), and a short fat wing which tends to generate a lot of induced drag at high AoA.
I would guess that the Bristol Freighter was similar to a motorglider - ***** heavy, with a fairly long and efficiently lifting wing. Not slippery however, definitely not slippery !
G