I think there is more to it then just small cell
There are successful treatment free large cell and successful treatment free small cell, and unsuccessful large and small cell both treatment and treatment free.
There are so many factory that play into this, bee genetic, food, temp, humidity, winter location, hive splits, swarming, break in brood... all have an effect on varroa. It can take anywhere from 1 to 3 years to kill a treatment free hive.
Varroa has hit the feral natural cell size population just as hard.
On that same note I am in the process of regressing to small cell, using essential oils and oxalic (as needed) until I get there. PF-100 to get there quick. And will also be using foundationless frames to cut costs. And if it doesn't work I will try different bees and stay with small cell.