Regardless of the enforceability of the document, the point is that the porn star/escort believes that he is bound to secrecy.
Moreover, I don't know that an NDA would specifically state that the escort must keep silent on the fact that they were hired for sex. It would be a terrible lawyer who drafted such a document. More likely, the language would generally address the escort's silence on meeting/interacting with the benefactor generally. Even if you hire an escort, you're not sleeping with them 100% of the time that they're in your company so an NDA which just generally proscribes mention of the meeting/interaction would be reasonably tenable.