Even if there were any corrosion inside the motor, you cant tell me it couldnt be machined out, or that most of it wouldn't be usable. Transmissions are generally pretty well sealed and so are rear ends. And I've never seen corrosion start on wheels over the winter when they haven't been cleaned all winter (non-chrome). Hell I have a junk bare block that has been sitting outside for 4-6 months that could prolly get away with a good hone if it didn't have a gouge in one of the cylinder walls.
Besides as mentioned before, in order for it to be considered a flood loss car, the water only needs to break the door sills. If the water only went that high, the motor would have no water inside. Some cars would have been completely submerged in water, but I would bet most haven't.