Bumps on the roof = was sitting on it at one point.
If it actually ended up resting with the majority of its weight on the roof it would have probably done more damage. As others have mentioned, you'd probably have issues fitting the windshield.
If it went down into the ditch and ended up mostly on its side it could have had some of its weight on the roof, enough to dent the sheet metal but not enough to bend the structure.
Still, I wouldn't buy this car. Maybe if it was dirt cheap it would be worth it, but the damage they're showing isn't enough to total it. To me it sounds like there may be an issue that they aren't saying, possibly a mechanical issue if they allowed it to run while it was on it's side/top.