I think it just means that there's only actually a single differential pair for USB 2 so connecting the two "D+"s from each side of the connector, and the same

I think it just means that there's only actually a single differential pair for USB 2 so connecting the two "D+"s from each side of the connector, and the same for the "D-"s, is recommended. This doesn't work for the super-speed differential pairs because they're actually different lanes.