Sorcerer inferno node only works with fire skills?

Not sure if this is a bug or intended, but the “spreading flames chance” from the inferno passive node seems not to work with non-fire spells. I was previously using firestarter’s torch and that procs spreading flames just fine with spells like lightning blast. But the inferno node does not. Does anyone know if that’s intended? The description doesn’t specify.

Sounds like a bug since the node doesn’t specify that. Does the spreading flames chance work with fire spells?