Holsters seldom actually dry rot, but the leather can become very dry.
I restored my 1918 marked holster, that had been crushed flat and dry as a bone, by first soaking in very warm water, then using a hammer handle to slowly reshape it from the inside. While the leather is still wet but begining to dry you should apply neatfoot oil. Applying neats foot oil to bone dry leather is a no no.
As the leather drys it pulls oil into the surface and the oil then locks moisture into the cells below the surface. It ends up as a even mix of oil in some parts of the structure and moisture in other parts.
Since the holster in question has been dyed black you can then seal the surface with boot polish. On black leather holsters I use a silicon wax polish.
The stitching of my 1918 holster is as good as new, though one of the brass rivets has all but disintegrated.
They used some treatment for the thread back in the old days, but I can't remember what it was. It was something other than just wax.