It is perhaps too simplistic to consider Melmetal the catalyst behind humanity's transition to the Iron Age. There is abundant evidence that humans at least attempted to work iron from ores and other steel pokemon before the 3kya event. The legendary Registeel is strongly associated with this era in Sinnoh, where no Melmetal remains have been found, and Melmetal's younger form of Meltan, with its voracious appetite for steel, is an archaeologically attested setback to metalworking in much of the ancient world.
Yet there is no denying that Melmetal, with the abundance of metal it produced from seemingly nowhere, led to the era of abundant weapons and Steelix clad for war. Those who accept the application of thermodynamics to pokemon have sought to explain this metal as the result of its appetite as a Meltan and some qualities of its digestive system in relation to evolution, but ancient accounts depict Melmetal as producing orders of magnitude more than such a process could possibly account for. Furthermore, much of the metal came out of Melmetal already shaped into useful forms, and votive offerings to Melmetal statues are ubiquitous in Iron Age sites thousands of miles apart.
However, it is hard to imagine the revolutionary societal changes of the period simply by pointing to the abundance created by a pokemon; Melmetal required no blacksmiths and no mines, and it is difficult to imagine wars fought for its strategic value in an age when its power dwarfed those of humans and domesticated pokemon alike. Although archaeology cannot effectively distinguish the two periods, contemporary historians have begun to date most of what we consider the "Iron Age" not from Melmetal's presence, but from its disappearance, and from the desperate efforts of ancient peoples to replace what they had suddenly lost.