Yes, the Potassium Iodide tablets only provide protection from a nuclear bomb or a power plant accident.
Radioactive Iodine is produced only by nuclear fission from a nuclear power reactor or from a nuclear detonation of fissionable nuclear material (such as Uranium-235). Following a nuclear detonation or an accident at a nuclear power facility where radionuclides are released in significant amounts to the environment, a person in the exposure pathway could be exposed to radioactive Iodine which may be absorbed in the thyroid gland.
Radioactive Iodine may also be produced for medical treatments. Medical radionuclides are usually short-lived so they deliver their internal radiation dose to the patient in a short period and will decay before they can pose a danger to the public from normal biological removal processes. Longer-lived radionuclides such as Cobalt-60 and Cesium-137 are used for localized radiation delivery, and never for internal exposures since they would eventually exit the body as waste.
However, a dirty bomb does not involve nuclear fission, but a conventional explosive that is used to spread radioactive material across a population. It would be highly unusual for a terrorist to steal medical radioactive Iodine for the purpose of using in a dirty bomb since only Iodine-129 has a half-life long enough to be a viable candidate.
Hope this helps you to understand the usefulness of the Iodine tablets. As I stated previously, nowadays most people already have thyroids saturated with elemental, non-radioactive Iodine since so many foods today are prepared using iodized salt or it is added before consumption. Ingestion or inhalation of radioactive Iodine would not concentrate in an already saturated thyroid.
On that note, Tollovir/Tollovid seems like the ideal treatment for all current and future variants of the Coronavirus. Vaccines currently being administered will likely provide limited or no immunity as more variants are discovered.
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