This doesn't surprise me, to be honest. Organisms have evolved to be fit enough to survive, and efficiency above a certain level might not affect an organisms fitness (or might even affect it negatively).
Was it input solar energy to O2 output? to H2 output?
Was it input solar energy to electricity output - trees doesn't directly produce it, so what thermal efficiency coefficient they applied to calculate conversion, if it is the case?
...
or something like that anyways..
Evolution optimizes by using heuristics and stochastic search--piling crap on top of crap, and obfuscating into oblivion.
Engineering optimizes by finding elegant mathematical solutions.
Sorry, getting off-topic. I guess I'm just bitter about not being a robot.
Good luck engineering a system like that. Elegant mathematical solutions are great, but only exist for limited, well-defined problems.