Well, "one is none" is not math, it's basically distilling a principle. It pertains more to Murphy's Law than math. And for anything which you consider critical (however you define "critical"), then it makes perfect sense.

If you consider fire starting critical, then having only one method available that can break, be lost, malfunction, get wet, etc. totally deprives you of that critical function. Depending on the conditions (including your own physical condition) you may have no other options besides what you brought with you.