I pulled a cup of water out of the jug, end of the day so minimum CO2 content relative to the morning anyway.
Measured the pH. Kept the vial.
I manually bubbled it a bit, just used a turkey baster, and let it sit for an hour or two.
Re-measured the pH and could easily see a lightening of the colour in a side by side comparison. It was not a full 0.2 but discernable.
My conclusion was that CO2 had essentially "in gassed" without being forced to with massive bubbling, I got tired of bubbling manually after about 30 seconds.
CO2 can't be lighter than O2, it just plain has more parts to it. In air, CO2 is close enough to the same density that it is mostly evenly dispersed unless you put it in a very controlled environment. In a tank there could be more CO2 at the bottom, which is why some feel concerned about their bottom dwellers, or any fish that go to ground to sleep at night while the CO2 is building up. With any filtration providing any circulation this is not really a concern.
In gassing and out gassing between water and air has more to do with the partial pressures involved and the affinity of water to absorb different gases to various levels of saturation. Ammonia can get as high as 500,000 ppm in water if the water is exposed to a pure ammonia source, CO2 is 1,500 ppm and O2 is around 40ppm from air... Pure O2 might be as high as 200ppm to compare against the pure CO2 source. Water has a higher affinity for CO2 absorption than O2 but oxygen makes up about 20% of the air while CO2 is less than 1% by volume (0.03% from one source) so there is just far less of it to capture even though it is easier than O2.