This is not quite true. Thiopental is fatal in very high doses IV. It was used as a single agent in executions in America. Until propofol replaced thiopental in anesthesia induction. Thiopental is no longer available in America. European manufacturers did not approve its use in executions.
5-6 grams of thiopental will cause sudden apnea, especially if injected quickly. More than enough to maintain apnea until 0% desaturation and death. Not only that, it is a very strong negative inotropic, just like amide local anesthetics. Presumably a fast bolus of this dose would cause cardiovascular collapse. The reason why it is used as a sedative before lidocaine in animal euthanasia is that lidocaine makes the process faster.