No dictionary is faultless.
And one should always bear in mind that the etymology of a word is one thing, and the meaning quite another thing.
If you found me sitting in a freezer, and shivering, you wouldn't stop to take my temperature before wrapping something around me.
Let me repeat this:
I very commonly have blood-sugar readings well below normal (below 4mmol/l), but I hardly ever have hypos at all.
So, either I'm an alien, or the definitions you quote are simply wrong. (Well, that's either or both.)