I think Timmy has had a VERY lucky escape. As I read your post I just knew you were going to say it was that substrate
Vile lethal stuff!
I admin a facebook forum for Tortoise Protection Group, with 17,000+ members & you would be surprised how frequently we have this problem occuring. With members desperate to warn others (& willing to share x-rays & vet reports)
Please steer clear of Tortoise Life or Tortoise Terrain, its harming & killing far too many torts & costing owners a fortune in vets fees