俱舍論記 17.18

17.18

「謂唯欲界至先不成故」者，答。謂唯欲界生得善根是其所斷。色、無色善未斷善時，先不成故，故非所斷。

Regarding "Namely, only the innate ... not having been previously possessed": An answer. It means that only the innate wholesome roots of the desire realm are severed by it. Because the wholesome of the form and formless realms was not previously possessed prior to the severing of the wholesome, it is not severed.