This has been settled for a long time. As far as the “visible from the moon” claim, just look at any of the Apollo photos where the Earth is visible from the moon. It occupies maybe twice as much of the sky as the moon does of the Earth’s sky. If the Great Wall were on the moon, could you spot it from here?
As far as visible from LEO, a lot of man-made structures can be seen from that distance. Highways, for starters. You probably couldn’t spot Hoover Dam from LEO, but you could pinpoint the spot between Lake Mead and the Colorado River.
Also worth mentioning is that implicit in the claim is that it can be seen with the naked eye. With a long enough lens and a sharp enough sensor, my house is visible from space — I found it on Google Earth.
A view from "outer space," on the other hand (as seen from Apollo 17 below), gives a much different perspective and clearly does not provide as much visibility for man-made structures.