Human intelligence is most certainly just optimization of response to external stimuli. The only reason why this might appear to not be so is because the shear amount of stimuli that we are capable of receiving and processing is beyond our current understanding. We can clearly understand what stimuli affects a cell, and so it looks like it acts in a programmed way. We don't know all of the stimuli that affects the judgment of a human being and so we erroneously believe that there is some special element to our decision making.

I am not saying that a cell is as intelligent as a human, or even that it has the same kind of intelligence; and I don't think the articles/research is trying to make that claim either. I am merely saying that one cannot call a human being intelligent while saying that a cell is just a machine carrying out its programming. A cell is no more or less programmed than a human. If one is intelligent to some degree, then so is the other. If a cell is not intelligent because it must operate within the confines of its capabilities then a human being is not really intelligent either.