You have a good point. I think that Ms Chiappa may have used the word “hardening” to mean heat treatment of the breech-block; thus, the implication is that the process of case hardening, which involves heating in a furnace to a temperature of 1,700 degrees Fahrenheit or more, is incompatible with the heat treatment. If I am not mistaken, the need for greater receiver strength at the turn of the last century, when smokeless powder was increasingly used in sporting arms, led Winchester to cease colour case hardening their actions (notably on the 1885 falling-block single-shot model), since they used a heat treatment process that was not compatible with case hardening.
I should think that Armi Sport could still provide the original look with a faux finish, and perhaps they will eventually do that. This, of course, presumes that the case hardened finish on the Spencers is not already a faux type, though it looks convincing and extends to the unseen surfaces.
I must confess that I, too, am surprised that a tougher breech-block is needed for the Spencer, given the characteristics of the .56-.50 black powder cartridge. Perhaps it is anticipated that some shooters will attempt to use somewhat injudicious smokeless loads. Only those Shiloh Sharps in .30-40 Krag, .38-55, and .45-70 are warranted for use with smokeless powder, but if these models have colour case hardened breech-blocks, then clearly such a block can be made that will withstand smokeless loads in relatively powerful (and high-pressure, in the case of the Krag) loadings (and presumably without a firing pin bushing, though I have not seen a Shiloh Sharps in these calibres). The decision at Armi Sport may have involved other considerations, as well, such as material costs, or the use of magnum pistol primers by shooters, which could batter the block. But I do think it significant that only the breech-block has been changed to a blued finish, on a gun with a plethora of colour case hardened parts. It hardly seems possible that this change was made for the cost savings involved in making one less part in a case hardened finish. This is the sort of topic that becomes more interesting, the more it is thought about........