The solution he explains is relatively simple: it involves creating a symlink for the folder in question after moving it to another disk or volume. After moving the folder, open Terminal, then type:
cd ~/Dropbox
Type ln -s, then drag the folder from its new location into the Terminal window, which will add its path; press Return.
This symlink lets Dropbox use the Camera Uploads folder, as if it were in the main Dropbox folder, yet the files are stored on the external disk.
You can do this with other Dropbox folders as well. For me, I use Dropbox's Selective Sync (Preferences > Advanced) to choose which folders get copied to my MacBook Pro, and have everything on my Mac mini, which has more storage.

