"...this hyper-partisan witch hunt" began in 2015 so the term doesn't seem accurate since the suit long predates his nomination as Secretary of State.  It didn't just pop up when he got involved with the Trump administration.

The suit is purely related to Exxon Mobil's alleged untruthfulness to its investors, shareholders and the public re:  the findings of its climate change research, and that Exxon Mobil has knowingly avoided disclosing business risks associated with climate change.  It's a corporate-transparency suit and has nothing to do with partisan politics.