The mining infrastructure didn't collapse. But tin was traded over very large distances,mostly from outside of that civilization's brotherhood of great kings (the rabbu surru system).

Iron bloomeries (via slag finds) appear in the record sporadically. It starts before the collapse, and may have very archaic uknown origins. But... they become common as recovery happens.

So... Causation seems to be backwards. Bronze production mostly ends. Iron working starts gradually expanding in the absence of bronze.

Also... The "superior weapons" angle isn't very strong. Bronze and early iron were not in a different league.

Iron just has more potential for scale, because the material itself is abundant. It requires a lot of labour and fuel, but if you can scale those you can scale ironworking. Eventually iron surpasses bronze and steel is vastly superior... but this takes many centuries.