This is correct in terms of being able to change your default profile location, however, please keep in mind that the profile folder isn't supposed to be synchronised or shared between different Joplin instances. "Filesystem sync" is what you're supposed to use if you want to sync files with synchronisation software like Syncthing.
iOS is very restrictive, so you cannot use any external location to sync your notes. In other words, you cannot use Syncthing to sync your notes with an iOS device.
On a related note: I'm trying to figure out how to find/change the file storage location for Joplin on my Windows 10 PC. I have two drives, an SSD and a larger regular drive – which is the preferred location since I have imported a large file set from Evernote. Is there a way to change/control note location on specific drives/folders after Joplin has been installed?