That’s actually not true - lens design on SLRs can be very compromised due to the extra distance to the film, which isn’t necessary on mirrorless since it hasn’t got a mirror.

This means mirrorless cameras have smaller lenses and can use all kinds of cheap older rangefinder lenses. People seem to think “SLR” means “film camera” though and don’t know that rangefinders exist.

What did I say that was not true? Despite what you've said, it's still true that a through the lens view is possible with shallower cameras due to the advent of the digital camera. Actually I don't see how your comment contradicts anything I wrote.

You said most of the rest of the camera can’t change, but lenses are part of the camera and can change.

I said, "most of the rest of the form is constrained by physics and ergonomics," and that still seems true? At a minimum, if digital camera construction has substantially changed the constraints on lenses, this fact has not led to a significant change in their overall form, to the best of my knowledge. If digital sensors substantially changed the physical possibilities, then perhaps it is ergonomics preventing them from changing.

You can see a comparison at the top here:

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.

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.