This is exactly what it looks like. [Oleg] calls it soldering in inert atmosphere, but it’s just a toaster oven reflow hack dropped into a container full of carbon dioxide.
Why go to this trouble? It’s all about solder wetting. This is the ability of the molten solder paste to flow into all of the tinned areas of a board. [Oleg] talks about the shelf life of hot air leveled PCB tinning, which is about six months. After this the tin has oxidized. It will certainly not be as bad as bare copper would have, but it can lead to bad solder joints if your PCBs are more than about six months off the production line. This is one of the reasons to use solder flux. The acid eats away at the oxidized layer, exposing tin that will have better wetting.
But there is another way. Soldering in the absence of oxygen will also help the wetting process. CO2 is heavier than air, so placing the reflow oven in a plastic container will allow you to purge air from the space. CO2 canisters are cheap and easy to acquire. If you keg your own homebrew beer you already own one!
If you’ve got everything but the reflow oven just look around for a few examples of how to build your own.