I'll admit I'm not well versed on the subject, but I don't think the vapor is just water, correct? Otherwise the pen would need to get insanely hot to turn it into vapor so you can inhale it. I've seen some pretty tiny vape pens. Doesn't make sense.
Does such a vape exist? One with 100% pure, nothing-else nicotine? No propylene glycol or glycerine?
It's been questioned whether that estimate is low, but the frequently cited LD50 for nicotine is 0.8-1 mg/kg, so by that metric a .7 mL Juul pod of pure nicotine would be more than a lethal dose.