Iodine tablets would give no protection whatever from a dirty bomb.
Potassium or sodium iodide is used to load the thyroid to prevent uptake of iodine-131 which is a fission product and has an 8 day half life, as well as I-133 which has a shorter half life.
No iodine isotopes would exist in a dirty bomb because it would have already decayed away. The only way to get atmospheric I-131 is from fission, i.e. a nuclear bomb or a reactor plant release.
So, I see three possibilities:
1. The Belgian government does not understand dirty bombs.
2. The Belgian government expects a fission bomb or a reactor "accident."
3. The Belgian government is distributing iodine pills as a placebo to calm the public.
Excellent commentary. A man who knows his science.
2 and 3 seem likely.
But it is hard to overestimate the potential of bureaucratic ignorance and stupidity.