[JJ_SO]

Huh? It makes no difference whether the camera body has a mirror or not, the field curvature in the image will be exactly the same. Field curvature does not impact AF either.

He claimed (correctly) that eye detection would focus on the eye regardless of whether the eye is in the center or edge. While the curvature might impact the overall image quality (for better or worse) them eyes will be sharp.

And what does sharp eyes have to do with field curvature? With a DSLR field curvature does not impact AF. The AF uses the same curved projected image too.
Weird "discussion".

"Weird" only, if one believes in accuracy of indirect AF and if one strictly ignores the full coverage of AF points, contrary to the comparatively small area covered in a FF DSLR - but even APS-C DSLRs will not reach the outer areas. And by using a fully covered frame, some compositions become possible you can't dream of to focus properly with a wide open lens.

Roger is talking about portrait lenses, some of them with pretty fast apertures. I often wished for more and better focus points in the DSLR, but that wish only gets fulfilled in mirrorless land. DSLR LiveView is more for static objects. If you don't need it, the discussion must sound weird for you.