Actually, according to the article, there’s no proof of radioactive elements being mixed in. There’s suspicion, but it’s not proven.
More importantly, it’s the kind of conspiracy that at the time would have been plausibly completely harmless to the conspirators. They could have and almost certainly did convince themselves that the material they were spraying was only a tracer and would cause no injury to everyone. Nothing to really cover up there - even if there were radioactive tracers in the mix, at the time it really wasn’t known how dangerous that could be. This wasn’t a case of ‘Let’s spray stuff on civilians and see if it hurts them!’. This was a test of dispersion of a tracer believed to be harmless of a time as a study of how well biological weapons would spread if an enemy used them on a US city.
Furthermore, it took a long time for harm to become apparent - it wasn’t like people were dropping dead the next day. This would continue to make it easy for the people responsible to convince themselves that no harm had been done. Actually, it’s still not proven that the health problems in the are even were due to the spraying. It certainly might be, and I wouldn’t be surprised to find out that it was, but it’s not a clearly proven cause and effect. This is why it took so many decades to be revealed, and it’s still not clear how big of a deal this was.
No conspiracy to commit harm beforehand, at at best a reluctance to admit what they had done when possible health effects showed up decades later.
Very different from 9/11, where the conspirators would have no possible way to believe that they were doing anything other than crashing airplanes into buildings full of people.