I have no idea where you read that, but I can tell you there is a lot of nonsense told about soft and hard Cotiucles.
For starters, there is the so-called "nail-test". It is said to try to scratch the stone with a finger nail, to determine wether it is soft or hard. I find this a dubious test. There are almost no Coticules that are softer than a fingernail, but most cases the fingernail will be abraded by the Coticule, which leaves a trace on the stone, just like a pencil would also leave a mark. Some people misread this as the fingernail "scratching" the surface.
When it comes to hardness, I can say this: of the entire Vault, there are perhaps 2 or 3 La Dressantes that I think are harder than average, and 2 La Veines aux Clous and one La Dressante that I would qualify as softer than average. Off all others, including all Laes Latneuses I ever tested, there is no hardness difference that I can discern with the methods known to me for making such an estimation. I can add that the hybrid sides indeed appear harder.
I have no clue what "butter feeling" means.