Talking to myself here but ... yet more ponderings...
NodeBB chat/pm seems to target the expected use case of users chatting actively whilst online, while making provisions for messaging and notification should they not. Yes, you can set preferences to also be notified via email, but those messages are still stored on server. Hence admins, law enforcement, etc. have potential to investigate should some whack job stalker join up and end up physically harming another member. I don't joke. It has happened. Fortunately not to me.
Contrast that with some boards that don't do real time chat in favor of piping message thru an email system for delivery. I am guessing to conserve on bandwidth and storage resources. This seems like a much greater potential for abuse by forum spammers cuz now they can blast entire forum populations w/o any push back until folks start checking email. Email has become essentially unusable for many such that they never check it unless they are expecting something. Like a confirmation or password reset.
Hence, it is pretty groovy that the nodebb devs were smarter than that. 😉