I think the point being made is that if you support Prop 13, you either a) are wildly misinformed about its effects, or b) don't care about its effects because you are in the privileged group that benefits from it at the expense of others.

(Whether or not the assumptions behind this assumption are true is of course up for debate, though I personally tend to believe Prop 13 is a bad deal, even for those who think they benefit from it.)