There is just no possible way that the federal minimum wage is going to be repealed, and even if it were, states are perfectly capable of setting their own minimum wage laws, as many do.
Friedersdorf seems to be suggesting that if a political change is unlikely, a politician's position on that change is irrelevant. That is unconvincing. It isn't easy to predict which previously off-limits political issues might suddenly be in play in the future. And in the meantime, voters are trying to make a decision about which candidate most closely mirrors their values. After party identification, candidates' issue positions are the best representation of that.
(Mistermix at Balloon Juice makes a similar argument.)