Their plan is to use the asteriod material to feed bacterias (it's hidden in the introduction, but explained in the main text). The advantage of using something more complex than air and water is that he bacterias can get energy from it, interad of using light like plants. I guess it's more similar to the method used to cultivate fongus.

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Anyway, you made me wonder if we can feed plats with sugar instead of light. Can we inject sugar into a potatoe and keep the rest of the potatoe plant happy in a dark place?

Anyway^2, I don't expect plants to be happy with the mix from the asteriods. It's probably some weird combination of small organic molecules. But some bacterias may eat et gladly and produce more usual organic molecules that we can eat.