This is very interesting, I never made this connection before. I also never knew about those ancient instructions for uprooting it. I am a firm believer that every myth is rooted in truth (pun intended), and the truth of the Mandrake myth is likely because the Mandrake root contains hyoscyanine which is a potent anticholinergic hallucinogenic drug that causes death in high doses. Dogs are even more susceptible to its' toxic effects. I doubt a human could die just by handling the root though, but if a dog nibbled on it a bit he certainly could.
But I wonder where the myth that the root screams when you uproot it comes from? Hyoscyanine causes auditory hallucinations, but to my knowledge they don't resemble a scream. It is very interesting to me that it is so prevalent in ancient literature and considered to contain "magical" properties. Just like the famous Amanita "fairy-tale" mushrooms, I'm sure the "magical" properties of the Mandrake root originated in its hallucinogenic effects.