While you're visiting this folder, check out the adjacent XProtect.meta.plist file. This file lists the date of the last modification of the XProtect.plist file and its version number. This is where the Safe Download Version application draws its information.

The updating of the definitions is handled by a Unix application named, appropriately enough, XProtectUpdater. You can manually run this file via Terminal. At the prompt, type: /usr/libexec/XProtectUpdater. This is what Safe Download Version does for you when you click its Update Definitions button.