It uses modified atmosphere packaging which basically means they replace the oxygen that would normally be in the package with gases that are relatively inert - nitrogen and CO2. According to the following table from the MAP wikipedia page, the parbaked bread is likely sealed in an atmosphere that's 70% nitrogen and 30% CO2.

I didn't know any of this when I bought the bread, but I was surprised enough by the prospect. Our loaves can be stored until late October.

They won't last that long though. We've cooked two loaves already, for garlic bread, my latest guilty gluten experiment. (These pictures are of the one loaf that's remaining ... the cooked ones don't last long.) While the resulting bread isn't better than fresh sourdough bread from decent bakery, it's a lot better than alternatives.

The brand is Essential Baking whose FAQ describes the tech as the following:

Anyway, I thought I'd share my little rabbit hole. My alma mater has a big ag and food tech department¹, I couldn't help being influenced by it, and to this day have an arsenal of pretentious wine knowledge I'll never use. Also, as one of the oldest technologies, it's nice to be reminded that food technology keeps getting better like anything else.

Extra fun fact: they keep prepackaged guacamole (like wholly guacamole) green, fresh, and preservative free with pascalization, which is like pasteurization, but uses pressure instead of heat.