quick google tells you that there are 187 quintillion gallons of water in the Pacific.
This is dumping about 613025 gallons of chemicals.
There’s a math of parts per million to go into, but it doesn’t take a genius to recognize that the amount contemplated is negligible amount.
The chemical weapons were dumped close to the shoreline, according to the Bulletin of the Atomic Scientists, March 1981 (available free online). Also, Sarin gas was tested on land:
U.S. Military secretly tested Sarin Nerve gas in Hawaii
Your calculation is certainly correct after complete dilution in all the world’s oceans has taken place, but neglects the reality that the concentration will be quintillions of times higher at the location of the actual leaks, before any significant dilution has taken place. These higher concentrations will easily be lethal to local life in the vicinity of the munitions.
Note also that if companies attempted to dump such chemicals in the oceans, they’d be fined millions of dollars for gross negligence. But, since the government did it, everything’s hunky dory.
But it would (will) take hundreds, if not tousands, of years to become fully diluted. If that is even possible.
Water exchange in the depths is very slow as there is no wind or sun to power movement as on the surface and in shallows.
In the short term seepage will be concentrated around the leaking containers.
More so at greater depths.
That's probably a good thing as the introduction of leaked chemicals into surrounding waters will be further slowed giving natural processes enen more time to dissipate the poisons.
Still, it is a shortsighted way to dispose of munitions and other dangerous substances.
You would be correct IF the dispersal were uniform over the entire volume. That is not how leaks happen. Coral reefs are very delicate habitats. An entire habitat can be wiped out with a drifting concentration of these toxic brews.