Is this android vulnerability bug #8219321 fixed?
Yes it was fixed (sourced from MasterKeyDualFix) on Wed Jul 03rd 2013. Generally vulnerabilities like this are reported to Google a long time prior to it being publicly announced. In this case it was reported to Google in Feb 2013 and the fix had already been passed to OEMs to include in their releases. (Note, the date it was committed to AOSP (the first link) is not the date it was available to Google and the OEMs)

As for the second part of your question I believe XMOD works by modifying the files after the app has been installed which bypasses the signature checks. Although I am not that familiar with XMOD I'm pretty sure that it is not using this vulnerability.