Shooting co2 into the keg and purging with gas alone does reduce o2 levels, but it doesn’t get as low as if you were to fill the keg with starsan and push it out. The problem with putting co2 into a keg with it open to the air is that co2 does not stratify very well. Gases will mix even though co2 is heavier, there is no reliable way to detriment if there is a “layer” over the beer. If you purge with gas alone with the keg sealed, the keg has to be filled and purge of gas at least 8 times before the o2 levels drop to a reasonable level and it’s still higher than a keg that was purge by pushing out a liquid.
Purging with gas is an improvement, but it wastes a lot of co2. By filling through the liquid post with a keg that maintains positive pressure, you get to the lowest level of o2 ingress a home brewer could expect and it’s about the same amount of work and time.
It seems complicated, but once you do it a few times, it’s not a big deal.