I don’t see him as narrow minded so much as well informed on the constraints we know about. Complex, multicellular life isn’t possible without an abundant supply of energy. And the most abundant supply of energy that we know is both workable for life on earth and likely to be available on other planets is the combination of reduced carbon (biomass) and molecular oxygen.

To dig a little deeper, energy for life only comes (as far as we know) from a particular type of chemical reaction called a redox reaction, which requires a reductant and an oxidant. The earliest life in earth likely used hydrogen or methane as its reductant and CO2 as its oxidant. These combos have major advantages in that they provide both energy and biomass. But they don’t provide very much energy. In fact, neither hydrogen nor methane when reacting with CO2 can provide enough energy to power a large multicellular organism. And in fact, as far as we know, oxygen is the only oxidant that can provide that energy. You can do the calculations yourself using the redox potentials of any pair of chemicals to find out how much Gibbs Free Energy is released when they react with each other. If you do, you’ll find that it’s really hard to beat oxygen and sugar (used as a proxy for biomass in general) without using exceptionally rare chemicals.