The point of comparing Mantle and DX11 on the same GPU is that it allows you to compare 2 APIs, all other things equal. Shocking, I know.
It is fine to add a caveat that this doesn't put DX11 performance of that GPU in the best possible light, but it's still a very interesting comparison. You can't have it both ways: "Look how amazing Mantle is compared to DX11, but don't you dare actually comparing them."
Even without game specific optimization, you could expect/hope a mature API like DX11 to not perform atrociously.