If you sign an NDA as part of an employment contract, you agree not to reveal publicly what you learn in the course of your job.
That’s a private contractual obligation, not a public one. It’s between an individual employee and their conscience as to whether they are a stickler about abiding by it, or whether they are a bit elastic about the obligations. Some people wouldn’t ever dream of breathing a word to anyone about the juciest things. Others learn confidential information, like JK Rowling really is the author of the Cormorant Strike books, and just can’t resist gossiping at a dinner party (true story).
Most people will mention some details, which they judge to be innocuous, to some trusted people because it’s hard to say nothing all the time to everyone!
You alos need to think about the consequences of breaching your obligations. The remedy is for the person you entered into the agreement with to sue you for damages. If you’re still in employment, obviously you can be sacked. Many employers won’t bother to sue you (well, they may not be able to identify you!) as, generally, it costs a lot of money and the horse has already bolted. What they might do is try and stop the confidential information being published by whomever their employee blabbed to.
Sometimes, your employer is not that bothered that you are breaching your NDA. Say, in the case of Meghan and Harry and Jason Knauf, because they specifically authorised you to share things covered by the NDA. Sometimes because they recognise that despite technically being a breach of the NDA, they can live with what has been put into the public domain. That might be the case here….