Yes, the issue with CO2 is how well it shields, but we're only talking about the electrode, not the weld pool. The Oxygen provided in the arc from CO2 or O2 in the sheilding when GMA welding is absorbed by the weld pool, and the metal being transferred to the weld pool, but they are not exposed to the arc for that long, so it is not enough oxygen to cause porosity when welding (unless you use too much O2, and then porosity will form). When you are GTA welding, the oxygen instead reacts with the electrode which is exposed to the oxygen for a large amount of time, and gets contaminated. The main difference though, is the electrode with GMAW is supposed to melt, while you try to avoid that with GTAW.