It is quite toxic, but a drop on skin being lethal is a bit of an exaggeration. If you get any on you, you probably have to talk to experts to address it properly. If you spill a lot on yourself it is indeed a medical emergency.
What happens is your body has to neutralize the acid that very freely absorbs into you through skin contact, and hours later when/if the amount you took exceeds your (mostly) kidney's ability to maintain pH/ion balances, your nerves stop firing for lack of calcium to enable the electrical activity and your heart stops beating normally, then at all.
We know how to treat it now, but originally it was, yes, quite a bit of bad news. So much so that it was used as an esoteric murder weapon in a mystery novel.
I think part of the, hrm, atmosphere of terror surrounding it is the delayed onset of symptoms, in the rabies "by the time you can tell, you're in deep kaka" sense. That and its ability to just slide through a lot of protection.
What happens is your body has to neutralize the acid that very freely absorbs into you through skin contact, and hours later when/if the amount you took exceeds your (mostly) kidney's ability to maintain pH/ion balances, your nerves stop firing for lack of calcium to enable the electrical activity and your heart stops beating normally, then at all.