It doesn't matter. The point is to show that light can induce a reaction to produce a more complex molecule. It doesn't matter where the particular reactants in this case comes from, and it doesn't matter if this particular reaction doesn't have quantitative yield. The point is that 1. The argument has been made that the second law of thermodynamics requires that light always break down molecules outside a living system and 2. Actual reality shows this is not true. You're once again shifting the goal posts--first it was "that's impossible," now it's "this particular reaction has crappy yield, especially in the presence of oxygen." Umm . . . different point, and one not half as compelling.
And I don't have to suppose anything, I know. :-D
It most certainly does.
And I am not shifting goal posts, the whole point is that a single simple chemical reaction which produces a more "complex" chemical is not life. Life is a tremendously complex arrangement of chemical reactions, involving non-spontaneous reactions. If the spontaneous reactions are observed in this complex arrangement, we call that death.