"There may have been more CO2 at the time but that is not what made more vegetation, it was the available water."
And there is evidence for this simply by looking at stomata counts in plants which lived in higher CO2 environments. They have fewer stomata, because stomata cost the plant in the form of water loss (and if they don't need them for CO2 intake, they are disadvantageous). The only reason plants perform better if they're placed in an atmosphere with artifically raised CO2, is because they have more stomata than a plant which is naturally adapted to that CO2 concentration.
