Not to detract from the central point, but that felt like too difficult of a "trick" to expect an 8yo to find.
I wonder if this was intended to be a "casting out nines" problem (check wikipedia if you haven't seen this). That can reasonably be taught to students that age and, if you know it, it's is easy to see the numbers given will always give 4 mod 9, and so 9000 is impossible.
I wonder if they had been taught this. (The fact that the student in question didn't seem to know it is not strong evidence, IMO)