The "has been subtracted at the unpacking stage" statement covers "possibility": if black pattern is not covered by black/cblack[], the only way is to subtract it on unpack to prevent LibRaw user from camera-specific details.
From user's point, there is no difference between 'black subtracted by camera' (e.g. old Nikons) and 'black subtracted on unpack()'
The "has been subtracted at the unpacking stage" statement covers "possibility": if black pattern is not covered by black/cblack[], the only way is to subtract it on unpack to prevent LibRaw user from camera-specific details.
From user's point, there is no difference between 'black subtracted by camera' (e.g. old Nikons) and 'black subtracted on unpack()'