It is a ridiculous exaggeration. Most baby boomers played with mercury as kids and some even swallowed it. Although mercury vapor can be very bad for you and it can even be absorbed through your skin, when creatures including kids swallow liquid mercury most of it travels through their digestive systems without being absorbed. The little bit that remains in the system can still cause serious health problems.
The liquid mercury in this situation is not much of a hazard because it weighs approximately 13.6 times as much as water and does not mix with the water. The hazard in this case is that the surface of the liquid mercury that is exposed to sea water will react slowly and form oxides and other dangerous compounds that are absorbed by sea life and enter the food chain. These compounds are a health hazard that cause all sorts of nasty problems and are most dangerous to brains and nervous systems.
As long as the liquid mercury that is now oozing out of rusting containers is covered with enough sediment the amount of mercury coming into contact with sea water should be minuscule and not much of a hazard, certainly nothing like Chernobyl. Comparing the two is “ridiculous” as you say.
You want a purchase 50 grams of mercury on Amazon for $50 including shipping. This makes it $454 a pound. But a standard 76 pound shipping container is around $2000 these days which is only $26.31 per pound. There are 143300.47 pounds in a 65 tonnes (metric ton or 1000 kilograms). So the value of the mercury assuming most of it is still in its containers is less than $3,771,668. The value of the mercury is not worth anywhere near what it would cost to recover it, and the risk of further contamination is probably far greater than just burying it as planned.
Recently a pkg containing a thermometer was damaged at the local post office...
So they burned all the mail in the building.