There actually are differences between dissolved CO2 and carbonic acid, but for the purpose of lowering TA we can pretty much ignore them. Several factors need to be taking into consideration for the rate of offgassing of CO2 and it gets quite complicated . CO2 can exist in the water at pH higher than 7.2 but for efficient offgassing having the pH low helps. Keep it simple since what you want to do is effeciently lower the TA. The method of dropping the pH and aerating works! Dropping the pH is what lowers the TA. Outgassing teh CO2 raises the pH without raising the TA back up. By having a lower pH you will have more CO2 in the water. If you don't lower the pH you can outgas CO2 and cause a pH rise but you have not lowered the TA, only raised the pH.