I don't disagree with you.
We all have a multitude of interests, self-interests, outside of the economic sphere for most of us.
The author takes it as a given that voters economic interests will trump all else. I dare to say he's thinking lower economic classes, but maybe not.
If Bill Gates acts against his economic interests to advance social causes, private or government sponsored through taxes, that's great. My guess there's a long list of the affluent the author would fawn over for their contributions in that sphere. But if we accept his economic premise that the Dems would be better for a middle class working stiff economically, he can't figure out how that working stiff would vote Republican on a moral basis when the Dems are offering an economic payoff.
A classic elitist, amoral view of the working class.