I'll go with the "No true Scotsman" defense here. Micheal Mann is not a scientist.
What we knew as science used to be rooted in the bible dictum to “subdue the earth.” God wanted it. Since God wanted it, it was also conscience bound to be honest, to represent the best reasoning that humanity could bring to bear, and dishonesty was frowned upon and its results discarded when discovered.
We don’t even have science any more here. We have something worse even than the SWAG.