The node description is targeted differently by default for different sized screens within XenForo. Currently you are missing device sizes smaller than 650px wide.
To apply the new css to all device sizes just surround the css with a media query...
CSS:
@media (min-width: 1px) {
.node.node--id13 .node-description {
display: none;
}
}
This is basically saying apply this css to all device sizes greater than 1px wide. Writing it this way will cover you in case XenForo in the future decides to target additional devices sizes.