Obviously I don't know their trade secrets. I can say, however, that for the M-Class there were dozens of engineers like you who thought that any US company could provide a Mercedes part as well as their German counterparts. The reality, however, was that the specified alloys (for aluminum parts) were not common in the US, and indeed the parts needed to be sourced overseas.
Why is that important, though? What would be convincing is if you could show an endurance test of different coolants in a running MB engine. I have a sample size of two where the water pumps started leaking after using "insert name here" coolant. Was it the ultimate cause - no idea, but I do believe in science and chemistry and corrosion, so I'm convinced (given the cost/benefit).

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Prost!