As bad as it appears, your horse got relatively lucky.
The damage missed the lateral extensor suspensory by a 1/4", slid along the top of the lateral collateral cartilage and probably cut into that cartilage at the caudal aspect of the structure.
It completely missed the larger stay apparatus and flexor tendon groups.
You may have misunderstood the vet. Ringbone isn't the concern here. More likely would be scaring then finally ossification of the proximal/caudal aspect of the lateral collateral cartilage, resulting in sidebone growth.
If the cut disrupted the coronary corium at the tubule origination point, the worst you'd see is a persistent quarter crack but is more likely to be irregular tubule growth and a weakening of the hoof at that point.