While true, the problem is it wouldn't meaningfully change the security situation for Iran.
Deliverable nuclear weapons make you invasion proof - nobody wants to risk it. A "dirty bomb" isn't something that can come flying in on an ICBM and eliminate large chunks of your nation - the threat of it is more likely to enhance aggression rather then deter it.
I'd say the same could be said for 9/11, which didn't really achieve anything positive for anyone but did make for a large bit of "The US hurt itself in its confusion"
Deliverable nuclear weapons make you invasion proof - nobody wants to risk it. A "dirty bomb" isn't something that can come flying in on an ICBM and eliminate large chunks of your nation - the threat of it is more likely to enhance aggression rather then deter it.