My understanding of the cold-rich start was not anything to do with the catalytic converter. Cold air is more dense and contains more oxygen. At the same time,

My understanding of the cold-rich start was not anything to do with the catalytic converter. Cold air is more dense and contains more oxygen. At the same time, when the engine is cold gasoline is harder to vaporize and doesn't vaporize properly, and also can condense on cold engine parts. Running rich while the engine is cold allows additional gas to offset both of those issues. When the extra un-combusted gasoline passes through, a secondary air injection system allows the fuel to be burned up before reaching the catalytic converter and does heat it up faster, but this is a secondary benefit rather than the primary benefit.