Well of course it cooled down. Adding more Co2 to the atmosphere doesnt cause the temperature to keep getting hotter and hotter ad-infinitum, it causes it to get hotter until the increased radiation from the Earth (due to it's now hotter temperature) balances the incoming solar radiation. What Co2 does is to cause a given rate of radiation from the Earth to corellate to a higher surface temperature. It's the same as putting a blanket on the bed at night. If the current temperature of the earth is hotter than that equilibrium point then it will cool, as was the case with that molten ball of rock at the start of the Earths history. Even with all those greenhouse gasses in the atmosphere the equilibrium temperature was way below the current temperature, so the earth cooled until it reached that temperature. This is also complicated by the suns luminosity then, which as best we understand was quite a lot less than it is now (the current theory iirc has it that the sun increases in luminosity gradually (about 6% per billion years) until eventually the hydrogen is depleted and it becomes a red giant) This means that for a given greenhouse gas concentration the equilibrium temperature will be lower (blanket example, if the temperature in the room goes down, even though you have a blanket on, you will cool down, Adding a second blanket though will help)
I gather that the current best estimate for the Co2 sensitivity is about 3 degrees per doubling of Co2 concentration. So if the average temp now is 14C and we double the concentration of Co2 then the average temp will slowly rise until it gets to 17C (and then stop if we don't add any more Co2).
However if instead aliens come along and microwave the earth, raising its temp to 50C, incidentally doubling Co2 concentrations in the process then the Earth will COOL gradually until it reaches 17C. But it won't go back to 14C again unless the extra Co2 is removed (or something else counters its effect, like the sun reducing in luminosity a bit)