Roasted salt is salt that is heated to some high, but undetermined, temperature. The salt I found claims it was heated to 800° C which is just about the melting point of NaCl, which I find hard to believe. As a chemist, I cannot for the life of me understand why roasting will do anything to salt. It doesn't modify the chemical structure in any way.
I'm wondering if they do it in a hot oven/fire, but that the salt itself doesn't reach the 800C. I imagine if there is any flavor change it's due to it picking up impurities from combustion. For example, smoked salt is a real thing (it picks up smoke condensates).
I have seen roasting a stick of bamboo with salt inside of it in order to have the salt pick up flavors, but this does not appear to be that.It seems to be roasted inside a sterile metal vessel. So it shouldn't pick up any impurities, save some metal ions. And that would generally not be desirable flavorwise.