"Bottom of the energy hill" is a chemist's short hand way of saying that we can't practically extract energy from CO2 reacting with anything useful, meaning chemically this is at the bottom end of reactions that happen without energy input. To convert it into something reactive, you need to invest energy to split it up again into its constituent parts, which is what happens during photosynthesis.
"Bottom of the energy hill" is a useful term in a lab. However the earth is not a chemistry lab. We cannot ignore photosynthesis outside of a lab as it is a factor that exists.
> We cannot ignore photosynthesis outside of a lab as it is a factor that exists.
I apologize if this wasn't clear. It's an expression about energy potentials, and as such it's not just useful in a lab, it's an essential piece of information about the substance.
For example in the original context of this comment thread, a person asked whether there was a danger of people exhausting our atmospheric CO2 because they got too greedy with this technology - a question that can in fact not be answered meaningfully without talking about energy deltas. The fact that CO2 cannot be practically processed in a way that releases energy is the only pertinent information when talking about this.
