The claim is that bronze enabled complex, highly centralized states because it was rare (due to a dearth of tin). So only the connected/rich could afford it, and they used it to build empires/kingdoms/etc.

Iron wasn't superior to bronze (in the fact the article claims it was worse), but it was good enough and widespread. All you needed was a furnace with bellows, charcoal, and an iron source (which were everywhere). Since everyone had access to iron, no one person/group had an edge over the others and this prevented highly centralized polities from being established.

Because other people learned to make iron weapons and fought back.

Bronze weapon technology had a moat because it required scarce, imported resources. Even if they learned the techniques, most people couldn't make them.

The resources for iron weaponry were readily available; they just needed to learn the techniques.

If only someone had written a whole article about this very angle, and posted it atop this thread...

Did you read the article? The argument was that tin doesn't occur in many places, so only the powerful even had access to bronze, which was a far superior material, which made it easy to subjugate the vast majority with no access to any material which could rival bronze. Whereas iron is abundant, so everyone has access to it, making it difficult to subjugate other people, because they could fight back.