The problem is that different people may scale their horses differently. To some, a 1 might mean completely lazy and refuses to move, so they'll rate their dead-broke but willing horse as a 5 (halfway between lazy and very forward). Some might think that 10 is only for psycho horses, and rate their very go-go-go horse a 5 (halfway between calm and psycho). Those two horses are rated the same, but have pretty different personalities.
Not to mention, some might use the scale only for what it says--temperment (1 for pocket pony, 10 for psycho?), others might use it for speed (lazy to very forward), other might use it for spookiness, as it seems you're inclined to think of it (1 for bombproof, 10 for spooks at his own shadow).
Add in the fact that it's possible to have a horse with a lazy temperment, but still spooks at everything it sees (at least it probably would spook with only a step or two, and not bolt all the way back to the barn?), or a horse that never wants to stop MOVING, jigs all over the place, but isn't actually scared of anything.
Better just go look at the horse, meet it, and ride it if you feel comfortable.
The owner saying it's always moving probably means it is jiggy, though, but not necessarily spooky. I rode a horse recently that was very AWARE of everything, but he never actually spooked. But he was VERY jiggy for the first five minutes, and still pretty forward for the next 15, until he settled in nicely.