Yes - serious water damage like flooding and immersion can be devastating for a car. But the above warning is a bit much. The Disco is made of a unibody aluminum construction so the concept of hidden structural rust in cavities that used to decimate cars from the 1960s and 1970s is totally irrelevant here.
Check for wiring corrosion if it got into the harness areas, fuse box etc. Otherwise id just fix the leak and move on.

 

Nobody will inspect the car though. I asked around ten body workshops listed on JLR website that they use for warranty work and none of them would look at the car. Only one spoke to me and said they won’t touch a car that it’s out of warranty. Perhaps they feel it is not possible to unwire every component from the roof, down the A pillars, and under the affected dashboard area and check each wire for corrosion. We do know that water is passing around the A-pillar air bags from JLR own independent inspectors report with moisture present around that safety component. Not a very comforting thought.