My understanding is that elemental mercury isn't really that hazardous. It's the compounds that are so very toxic and also I guess vapor. But I don't think that at room temperature vapor is really a problem, only in industrial settings in the past where hot mercury was used, mercury compounds, or other compounds that were simply contaminated with high levels of mercury products. Like a lot of hazards, it's probably overblown, assuming you don't try messing with it in a chemical way and end up with organomercury compounds.
Agree, it's not really that hazardous in an elemental form. You can find medical reports where individuals ingested or injected mercury into their skin; it can take weeks or months for inflammation to start and after removal the toxicity treatments are relatively straightforward.
I dunno if anyone really cares about that, and if I were a conspiracy person I'd say it's a cover story. What they do
care about is what mercury does to an aluminum airplane -- note that they list it as a corrosive
. See: http://periodictable.com/PopSci/2004/10/1/index.html