They mention high mercury levels as a trap or from wine that the emperor drank, but neither is likely (they often used to add lead to wine as a sweetener, but not mercury AFAIK). But, mercury contamination in tombs, especially in Asia, is very common from the heavy use of the deep red pigment cinnabar, also called vermillion, which is mercury sulfide.
Faik the worry isn't from wine the emperor drank (though I think the consensus is he did take a hell of a lot of mercury as medicine believing it'd give immortality). It's the described artistic floor map of China (at the time) with the rivers of liquid mercury suggested as being real by the high mercury readings.