Here's a video where John Carmack, Tim Sweeney, and Johan Andersson (from Dice) talks about AMD Mantle in 2013: https://youtu.be/MH2hjhcfWic?t=4975 Carmack is sceptical, Sweeney likes it but doesn't want yet another API.

And here's another video where Andersson talks about the motivation behind Mantle: https://www.youtube.com/watch?v=KApdf4P2Iak