Of course an admin can always open up the database and read your forum PMs, that's not surprising. The very first line in the link you provided, however, is what I was worried about:

> Moderators can read PMs that have an active flag.

This system is now setting nsfw flags in an automated fashion, specifically seeking out content that the persons involved wouldn't want others to see. Clearly a forum is the wrong place for that content, but people don't always make good decisions (especially kids; I was a kid on forums too and would be very surprised if nothing ever transpired there). The receiving person can already flag anything they deem inappropriate. A system making automated decisions about messages that were intended to be private creates problems and it is not clear to me who this serves