Not so. Brake rotors are not hardened. They are usually made of ferritic gray iron.

The melting point of a material is only important for the foundry, not for rotor performance. Pure iron has a much higher melting point than carbon steel or cast iron, yet it is soft and has little strength, so melting point has nothing to do with metal performance.

It's rather hard to screw up cast iron metallurgy as it relates to rotors. The chemistry control for gray iron industry is not tightly controlled, nor should it be tightly controlled. Gray iron is produced to specified grades of minimum strength and ductility limits. Rotors are the least demanding of gray iron material properties. The only stringent requirement I remember is that max carbon flake size should be 0.01" MIN so that brake rotors won't be noisy. Most applications have a MAX flake size requirement for strength. It's usually poor foundry practices that result in a bad product, i.e., porosity, improper shakeout, and bad sand control. I've worked in the foundry business and qualified product for the automotive industry.

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95 E320 Cabriolet, 169K

Not meaning to disparage or step on your obvious experience,

So, it's hard to screw it up, or not??

I've always believed that noisy brakes are usually caused by the metal backing on the brake pad chattering against the metal brake piston under pressure and friction, not by resonant rotors. Perhaps if the pads and rotors get very glazed, that might cause some squealing from the pad/rotor surface.

Regarding what a layman may generally refer to as "inferior metallurgy":
I've not worked in a foundry, so I surely would have to bow to whatever experience you have there, but regardless of standards for certain grades of iron, I would still have to think that the quality of the overall manufacturing process, including any alloys, fluxes, retention of impurities, annealing/tempering if any, stringent quality controls like batch testing and control of the mentioned flake size/structure and stress, and the quality of the final machining of the finished product would all factor in to some extent for the propensity of any particular manufacturers product to warp under various conditions. While the melting point may not really have an affect, many other things likely do that would vary from maker to maker.
There are most certainly great differences in quality and (quality control) between brands and manufacturers. If an iron rotor was merely an infallibly produced hunk of metal, all the rotors we buy would come from the cheapest sources available.