The ADL has been successfully sued for defamation before. Back in 2000 the ADL lost $10.5 million for defaming a Colorado couple:
https://jweekly.com/2000/05/12/judge-fines-adl-10-5-million-...
"On April 28 [2000], the jury found most of the charges leveled by the ADL, based on the tapes, to be 'not substantially true.'"