I'm not sure the discussion has touched on the reason people are making that claim about rangefinder and/or mirrorless cameras. Those discussions all seem to address the retrofocus lens design necessary to get wide angle effects on an SLR. A retrofocus design being necessary because the rear element of a wideangle lens without the retrofocus group would be too close to the film plane/sensor and interfere with the operation of the mirror. The claim is that the retrofocus design introduces more barrel distortion. However, I expect modern lens designs have done a good job incorporating additional lens groups to counter that barrel distortion.
I really have no idea whether these claims are valid or were ever valid. Certainly, if you can eliminate the retrofocus group and any additional elements necessary to counter barrel distortion, the lens can be smaller/lighter, but would that really give them any advantage over a well designed (albeit much larger) DSLR wideangle lens with respect to barrel distortion?