I'm not 100% certain that Sleheyron is the volcanic world. I'm more inclined to think it's the barren world. The Ebon Hawk's galaxy map has a description for Korriban that says, "Terrain: Volcanic Mountain Range".
We see Korriban as a very mountainous place, which can very easily be indicative of volcanoes, active or dormant. Not necessarily the type of lava planet that we see in Mustafar, but lava nonetheless. I could go even further to say that the deep passages in the crypts and caves explored on Korriban was possibly partially filled with lava at one time, but that's just speculation.
Sleheyron, on the other hand, seems to be a city build on fairly flat ground, which is typically not associated with volcanoes or lava. In my opinion, the only reason that Sleheyron has been associated as a volcanic or lava planet was because everyone assumed Korriban was the barren planet. And the evidence in KotOR seems to point to Korriban being the volcanic planet, despite the lack of visible lava.
We're planning on leaving this somewhat ambiguous so that the player can draw their own conclusions.