It seems like it's not a microcode update fix, but an MSR killbit flip from the BIOS? [1]
[1] - http://support.amd.com/TechDocs/41322_10h_Rev_Gd.pdf page 66
[1] - http://support.amd.com/TechDocs/41322_10h_Rev_Gd.pdf page 66
(While it doesn't apply for this specific example, it's still the reason why one generally will see microcode-fixed errata go away rather quickly, independently of BIOS updates)