We prove the open question posed by Zhuang and Hu in Remark 3.1. More generally, we consider symmetric joint probability mass functions and joint densities whose associated quadratic form is non-negative. In this class, for every \(r>0\), the inequality \(\E\abs{X+Y}^{r}\ge \E\abs{X-Y}^{r}\) holds for all distributions with finite \(r\)-th absolute moment if and only if \(0<r\le2\).