The comparator offset doesn't affect the voltage division of the CDAC, so it is as before without the offset. But it affects the decision of the comparator. For example, flipping the MSB cap to Vfs is supposed to compare Vin with Vfs/2. But since the comparator has an offset, it will compare it with something else. Say the offset is 30mV and the input is at 0.52*Vfs. The CDAC produces voltage of Vfs/2. Without offset, the comparator will indicate that the input voltage is higher than the reference Vfs/2. But with the offset comparator will say input is still below. And this is valid for every input voltage.