It is not called a steam explosion when the graphite moderator block explodes in fire. Obviously in such an explosion a lot of steam from the cooling system is created.
What might have melted down, was remains of the reactor: after most of the inside got spread over all the place.
If we say: "the reactor melted down" we consider it the main incident. Not an aftermath of the main catastrophe.
No one cares if there was enough left of the reactor or fuel that there could melt down something after it exploded.
Fukushima "melted down" after power loss, due to the tsunami, and steam explosions wrecking the reactor vessels ... however: the main disaster were the melt downs.
Big difference. Hence Chernobyl was a gigantic catastrophe, and Fukishima only polluted 1/3rd of Japan, lightly (arguable).