[archgoon]

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.

[strainer]

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.