You are correct saying that CO2 in the form of dry ice doesn't usually have a liquid phase. Not that that matters much because dry ice does not contain any water and does not speed of support decomposition.

But the air already in the contained certainly has some percentage of water. And when dry ice is introduced the temperature into the air around the dry ice drops, which causes the moisture in that air to fall out and form condensation. This condensation isn't going to be forced out by the CO2 liberated as the Dry ice sublimates.

If you could introduce the CO2 into the container without markedly dropping temperature, you would better remove both oxygen and moisture.

Is that a desirable goal; depends. If you are packaging food in a low humidity location, a northern state in the winter, it might not make any difference because there isn't enough humidity to worry about. On the other hand, if you are packaging food in the south in summer, with humidity pushing 100%, it may make a big difference in how long the food lasts.

That dry ice has been used for years like this isn't much of an argument in my book. So far most people using this technique are reporting they get 'good' results. After a few years the food comes out in good shape. The question is whether this is as good as it gets. If they get good results at five years who is to say they might not get ten years if they used gaseous CO2 instead of solid CO2.

It also has to be noted, based on informal search on my part, that many of those using dry ice are in states with presumed lower relative humidity. Methods that work well in Arizona might not work so well in semi-tropical south Florida.