So called "Dropbox Backup" feature is in fact automated move of target folders into Dropbox folder and set links (references) on their initial places. Only files/folders residing in Dropbox folder keep sync! Actually this isn't backup, but sync. 🤷
Indeed the automated folder move is limited to only few folders, but you can do the same for all folders, whichever you want, by hand (as before this feature gets come up). 😉 Mentioned feature is just automatization of a procedure used before as a replacement of the support for already deprecated links. Not very adequate. That's it.