But the result wasn't discredited: there was in fact a normalization error in Mann et al's original findings, and we didn't find out until 8 years later because the data manipulation code was "proprietary."

Meanwhile, everyone proceeded as if it were fact.

We need to raise the bar of skepticism and with it the burden of proof. Show me your raw data. Show me your code for manipulating that data into the final result. I'm honestly surprised that publishing data + paper + a source code repository isn't required in order to be taken seriously these days.