No, He/She is using a symlink.
A symlink is kind of an alias, or pointer. It allows a single folder to be shared between multiple places. It's a thing in NTFS and basically all UNIX like filesystems. *Nix flavor OSes and BSD flavor OSes use it pretty aggressively for system libraries and the like.
In this example, the dropbox hosted file share is attached to both systems using a symbolic link. The file is never copied, it is just accessed from both machines. Something windows users may be more familiar with would be a windows file share, such as on a NAS. The deal is that the folder in the share is mapped into the local filesystem using the symbolic link. The dropbox folder LOOKS like a local folder, but is still physically on the dropbox server. Copying never happens.
The issue might be that the mac is using filesystem caching, and the contents of the filesystem changed behind its back. It is expecting a local disk after all, which should not have its contents changing mysteriously. Another issue might be date/time related, if the clocks of the two systems accessing the share are not synchronized. This may make files that are "Not legal" because they have a creation date in the future, as seen from one of the system's POV.
So, I would first make a backup of the save, then issue a sync command on the mac to make sure that the filesystem buffers are clean, make a test file on the share using the mac, delete the test file, then see if DF sees the saves again. If it doesn't, then look at the system clocks of the two systems, and the creation/modification dates of the files in the dropbox share. If the systems dont have matching clocks, point them at a conserved NTP server. There are several public internet ones. That should keep the clocks in sync so that files created by either should not be "in the future".