https://www.kenrockwell.com/tech/rangefinder-vs-slr.htm
Sony sells wide angle "pancake" lenses that beat the ones for SLRs at compactness:
https://www.backscatter.com/Sony-E-20mm-f-2.8-Pancake-Lens
https://www.bhphotovideo.com/c/product/12082-USA/Canon_2509A...
Though there's some small SLR lenses too, I think they distort more or it takes longer to figure out the optics - not an expert there.
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.