I don't think Hindenburg is scared of being sued, otherwise they wouldn't have been in the business in the first place. Their investigations are extremely thorough, with everything fully documented, mostly from public records, for the specific purpose that if/when they show up in court, they can be confident of the basis of their findings.
https://www.reuters.com/legal/government/adani-group-threate...
P.S. where did you see Block suing Hindenburg? The closest thing I can find is 'Block said it intended to “explore legal action” against Hindenburg, who sought to “deceive and confuse” Block investors to “profit from a declined stock price.”' which is just PR speak