Okay. Someone has to say it.
The Leitz lenses were highly corrected for Chromatic Aberration, more so than the Zeiss, Nikon, Canon, and others. Look at a collapsible Summicron, Summarit, Summitar, or SUmmar and note where the Infrared Index is. You'll find it at about the F2 mark. Most lenses have it between F4 and F5.6. When shooting visible-only, that means all of the colors fall closely together, very little dispersion. When shooting B&W, it means less smearing. Take that, and the lower contrast of an older Leitz lens, you get better (in terms of more shadows detail and less blown highlights) results from B&W images. I almost always shoot B&W with the older Canon 35/2.8, Schneider Xenon 50/1.9 on the Retina, or any of my Leitz lenses. All of mine are older. I do not get the results that I like when using the Nikkor lenses with B&W. The lens that I use on a Nikon RF for B&W is a Menopta 53/1.8. It is lower contrast, but not as well color-corrected. Someday, I'll hack a Summicron into S-Mount. Use the removable head and it's bellows adapter.
It all comes down to the Optics, and how the optical engineers optimized them. The difference between Zeiss and Leitz lenses is well-known, and you can find it described in older Photographic references. It's not a myth, and can be described in optical terms.
Some of the problem is the terms used to describe the difference. And if the difference is for the better or for the worse. I like shadow detail to be preserved, and highlights not to be blown. For that reason, I prefer a lower-contrast lens for B&W work.