They probably were swapping out the gen2 shortblock to a gen3. If the gen3 was ordered new it would not come with an oil pan. So to make everything work with what they had they just reused the pan.
Yes it physically bolts onto the block, but it's not the only thing that needs to be swapped out in order for it to work. If you have the engine as a bare shortblock, yes it's easy to bolt on. If you have the engine in the car, you can't just drop the pan and put a new one up in it's place. You need to do alot more work.A gen2 pan is a simple bolt into the gen3