Alkaline buffer is pretty much baking soda (NaHCO3 → sodium bicarbonate) mixed with a little bit of other bicarbonates (potassium, magnesium), but the main ingredient is baking soda, so there is no practical difference between using Alkaline buffer from Seachem or baking soda to raise KH.
Regarding the effect on CO2, I'm pretty sure bicarbonates doesn't interfere with the amount of CO2 you are putting into the water in terms of lowering it. Once the CO2 you add is dissolved, it will combine with a water molecule and release a proton H+, turning into a bicarbonate it self. That proton can stay free lowering the pH or being captured by a carbonate to transform into another bicarbonate preventing the pH to going down.
Only if the pH falls to bellow 5 most of the bicarbonates will turn into carbonic acid, so, increasing the amount of CO2 dissolved even more. If you keep putting acid into the water (H+ ions) all bicarbonate/carbonate will be transformed into carbonic acid and will be loss as of CO2 in gas form.
Adding baking soda to raise KH from 2 to 4 will only affect the pH given the specific amount of dissolved CO2. While 30 ppm CO2 in KH 2 will result into a pH around 6.3, with KH 4 the same 30 ppm CO2 will give you a pH 6.6.