Wild guess, touching this with a 10-foot pole risks validating his claims. If they sue for breach of NDA, it means his claims are factually correct, and if they sue for libel and it goes to court, they may be forced to submit documents they don't want to.

Most likely, the author was let go in mass layoff, and they forgot about NDA.

If they can swing it as legit whistleblowing somehow, they might be ok.

Interesting point. Time will tell.