Sharpness has nothing ado with depth of field....
And even wide open, or at least at f2 central sharpness on the Sigma is quite impressive.
So yes the sigma is sharper than the 135L.
It is funny though how people complain about the irrelevance of a sharpness test, while they are actually complaining that their lens is better...
Both are telephoto portrait lens, so that's why DxO compare them.
Sharpness is not the most relevant factor for portraits, that's a given, but DxO does not pretend to test usability or practicality of a lens.
DxOMark tests resolving power, vignetting, light transmission and so on.
A kind of pure image quality test, be it relevant or not to the application.
If you want field tests and reviews, there are plenty of sources such as SLRGear, DPReview, photozone.de... and so on.
DxOMark is just a metric...