Hamermesh downplays the real game theoretical reason why it’s rational for Han to fight: His contribution is likely to be decisive to the outcome. After all, he’s got “the fastest ship in the galaxy,” and it can make mincemeat of Imperial tie-fighters (as we already saw earlier in the movie). Hamermesh’s payoff matrix implicitly represents this by positing that if Han fights, he increases his own payoff from 5 to 8, and that of the Rebels from 7 to 10. In truth, however, Han’s contribution might well make the difference between victory and total defeat (as in fact happens). Moreover, the speed of the Millenium Falcon minimizes the risk that Han takes should things go badly. He has a good chance of running away unscathed. I’ll ignore the fact that he also times his arrival at the battle perfectly, such that it’s clear exactly what he has to do to ensure victory at little risk to himself; if it looked like the Rebels were going to lose, he could have just as easily have destroyed Luke’s fighter instead of Vader’s and then claimed he was there to help the Empire all along.
16 hours ago