Interesting.
How are they certain that these elements didn't leach into the human remains during the centuries after their interment? What if these compounds were somehow used in embalming?
Not knowing a d&mn thing about forensic archaeology, I couldn't really say, but it sounds like there might be alternate explanations.
Vasili III was the father of Ivan the Terrible, and Anastasia Romanova was one of Ivan's several wives. Poisoning played an important role in palace turnover in those days, apparently.