Depends on various factors but not necessarily. It depends on if the blow were concentrated on one area of the skin (more likely to bleed) or spread out across a larger area (less likely to bleed). Depends on the injury caused (a neck can be broken with no signs of trauma on the skin surface, for example). Depends on what the victim was wearing; since Kyron was wearing a T-shirt, I don't think this was a factor that would have prevented bleeding.
Also depends on how and where the victim fell.
I've been hit twice in parking lots (I'm deaf in one ear and unable to localise sounds); both times I was knocked off my feet but neither time did I bleed.
I would expect that there was some damage and maybe even blood on the vehicle, but since it was several months before LE got around to figuring out where various vehicles were in the parking lot, that particular vehicle may not have been identified as having been in the area that particular day.
If the vehicle damage was limited to scratches or paint chips, it may not have even been noticeable without a forensic exam. Which was never performed, of course.
I think it's a long shot theory but at this point, I'm thinking that whatever happened to Kyron was a long shot or it would have been solved by now.