By dangerous, I assume they don't necessarily mean death...but, other complication of low blood glucose level..such as tissue damage...and, also I think, PPH overestimates the amount of lethal dosage...because, they dont have all the information, individual sensitivty, hepatic/kidney function, body weight is a strong determinant of lethal dose required...also, mode of administration, there's a difference between 300iu IM and IV...its also genetics, human body works in mysterious ways...I have heard of people dying with a way way less dose of amitriptyline, than as advised in the PPH...and, others making a complete recovery after massive overdoses..
But, I think, 1000iu insulin IV is fairly lethal, the only problem is administering the complete dose before falling into a coma.