Inferno bug?

So I tried making an inferno Renekton in a tft match. I am not sure if this is a bug or working as intended but Renekton's ability cause does not cause fire to spread on the ground. Is this due to it dealing physical damage? Does anything dealing physical damage through an ability not proc the fire? If this is the case of it being intentional then perhaps a rewording of the inferno origin is in order stating the magic damage causes the flames.