Ken is mistaken about the size comparison, at least between that Leica and the Canon lens. The Canon is a wide zoom. Zooms with autofocus are of course going to be much larger than a manual focus, slow 21mm prime. (That prime is 3x as expensive, two stops slower, and lacking an autofocus motor compared to Sony's similar offering: https://www.bhphotovideo.com/c/product/1126141-REG/sony_sel0....)

I will also note that pancake lenses have been around since the sixties and I believe that there were pancake lenses compatible with DSLR lens mounts.

Suffice it to say I'm not sure I can say I am convinced DSLR optics are disadvantageous vs. mirrorless optics. I wish Ken would cite some sources because on Google his claim is the first and only relevant one document that turns up for a few pages.