I'm fairly sure BestDef was added very late in the 1.10 beta. Vid's first post where he notices BestDef exists is from the 12 of July, while he'd been working in those files and reporting xml bugs in them under 1.10 beta in the week leading up to release and so would have noticed it earlier if it had been there. I've certainly not seen any sign of BestDef doing anything different from its current behaviour, and its name does strongly imply that it's present form is correct.
Given Vid and I probably fiddled around in that file more than anyone (aside maybe from TurielD), and both of us suspect that the problem comes in the moment BestDefense shows up, I'm inclined to believe it appeared with 1.10 rather than it working correctly in 1.10 and then being broken by 1.11 - especially since the 1.11 changelog contains no references to any alteration in best def, or anything related to it, and the patch came out only 24 hours afterwards.