Meaning of a reaction being "too slow to be biologically significant"?

[asimov42]

I'm wondering if there's a clear definition of the phrase "too slow to be biologically significant"? (in this case the reaction of the auto-oxidation of $$NO$$ to form $$NO_2$$) Does this simply mean the reaction occurs with a very low probability? Presumably the reaction does still occur - I'm wondering if there's a clear technical defintion.

Thanks.