Old thread. But (no pun intended)…
It’s always been my understanding that when cyclists refer to calluses on their butt, it isn’t in reference to skin that has hardened on the outer layers. I’d call those saddle sores. It meant that the interior muscles and tendons that your sit bones are in contact with when you sit on a bicycle seat had become callused, thickened, or hardened, and are able to withstand the long periods of the pressure from your sit bones.
I don’t know if this is by any medical professional, but here’s an explanation —
https://bicycles.stackexchange.com/q...rious-cyclists

Dan