The theory holds that both bodies were in a molten state after the collision.
Molten might not mean hot. The bodies tend to slosh around as if they were liquid because of their size. Even during earthquakes the solid land waves as if liquid and the ground might even liquify altogether allowing buildings to sink. The liquidity of the apparently solid surface is shown clearly by land tides that occur just like ocean tides and the movement up and down is about three feet every day and this without vibration as would be associated with earthquakes of similar motion.