Sure, mammals existed before the big thump, but did they exist in the same numbers, in the same environments?
No, I dont think so.
The meteor impact had the effect of knocking the top off the food chain, leaving room for more adaptable creatures to fill in the gaps. Already being covered in hair, mammals succeeded where the majority of lizards failed once the "nuclear winter" effect kicked in. The sudden increase in global temp would have come about as a result of the dust in the atmosphere finally settlnig back to earth, allowing the suns rays to get back to warming us up.
If you apply this logic to the modern day, its like saying that Californias Zero Emission Policy has absolutely nothing to do with the increase use of bio-fuel, fuel-cell, and electric vehicles in that state.