The original definition of “magazine” was simply “warehouse”. A place where you amassed a bunch of stuff. This usage is still around, but it’s rare.

At the time, the military was perhaps the most prominent entity creating these well-stocked warehouses, so the specific linking of “magazine” to “military warehouse” was a natural progression. And as we all are familiar now, it later morphed into a word to describe a chamber of bullets. In a sense, a tiny little military warehouse you attach to your gun.

The definition referring to a paper catalog getting mailed to your house full of random written articles comes from one very specific one, named Gentleman’s Magazine. It was named that because it thought itself a magazine (a warehouse) of information.

I assume kbin was thinking of the latter when using the term to describe its communities. Though, considering the right-wing bias of its target audience, I expect the wordplay with the ammunition definition was also intended.