I'll bet the nucs thye make up survive their first year with or without treatments.
It's not all about a brood break .
When you split a hive up into nucs you are taking the mites in the brood and divvying it up throughout however many nucs you are making up so you no longer have a hive full of mites you have 10 nucs with one frame of mites. Add a brood break to that if you are using queen cells and you have even less mites.
The new queen starts laying like a machine and the bee numbers increase dramatically much faster than the varroa can increase so mite problems are reduced.