Oh, that guy. Suing PNAS and the author of a study critical of his work is honestly quite shocking. One would hope the work would stand for itself, or at least the response should’ve been another paper rather than a lawsuit.
It is thoroughly inappropriate to respond to legitimate criticism, made in good faith, published in a reputable journal, with a lawsuit.
It doesn't matter if you're right. If you're right; arguments can be made to show that your opponents are wrong (and he was in fact allowed a rebuttal letter in PNAS). Filing a lawsuit breaks any semblance of civility of discourse and dispassionate pursuit of truth; and results in everyone worse off, as the discussion has become thoroughly toxic.
It's burning down the entire house because someone disagreed with your analysis.
Whether it was appropriate or not depends on whether the criticism was legitimate or not. That is the legitimate purpose of legal proceedings to determine.
I guess you would be a heretic to be someone who complains that while the rich want to pay for renewable energy and can afford to live in such a Friedman-esque society in which money buys your civil liberties from the legal system--or even just protection from criticism... meanwhile cost of living, suicide rates, homelessness, civil liberties continue to deteriorate for the working class.
Edit: maybe the downvotes suggest my guess was right.