They aren't planning on suing for a breach of NDA. They're planning on suing under defamation law, which alone will cost the former employees tens of thousands, if not hundreds, to defend; if they can't defend their case - and as the article says, opinion is not automatically protected under New Zealand law - it appears that they will be suing for "increased recruitment costs", which sounds like it could be construed to run to arbitarily large numbers.
The point is to terrorise anyone out of criticising them, ever.
They have 5000+ employees. I don't think their recruiting efforts have been hindered. The CEO's decision to pursue this has a larger negative effect than the comments on Glassdoor. I'm evidence as I had never heard of this company, do not peruse Glassdoor, but now I know this company's name and that the CEO is a tosser.
The point is to terrorise anyone out of criticising them, ever.