As someone who also (in my case totally inadvertently - i was introduced to Diplomacy by friends who liked to play it and we often sat in games together and had no idea such a thing was "bad" until I was told by the mods) taken to task for metagaming and essentially told I could no longer play in games with anyone I knew in real life unless we all announced it to the entire game beforehand, I would also say this rule is a bit unclear.
As it is now the rule is "Do not play with real life friends if you cannot play fairly with them. End of discussion." but what does that mean? In my case I think that if RL relationships influenced decision making it all it tilted us towards stabbing each other, trying to beat each other, etc. But the fact is that millions of things influence my decisions. I have stabbed someone because I read a bunch of obnoxious political posts of theirs in the forum. Meta-gaming? Absolutely. But I had to choose to stay with one person and stab the other. One reason had to tilt me in one direction or another...
But, and here is where it gets tricky, Diplomacy IS a metagame. It is the meta-est of games. It encourages and requires you to get as much information as you can by any means necessary.
And people refer to past games all the time. How often have you been told in press "Look at my history with country X, I often work with country Y" etc. That is also metagaming.
And I don't understand the privileging of RL relationships over other ones. In fact, isn't it MORE likely that two people here that are regular diplo players and have a history of playing together will be making moves based on others tendencies in very real ways than just a vague sense of "Maybe these people will softplay each other because they are friends IRL" (and really, you must have some weak friendships if they can't handle a few knives in the back, or even relish them) I would MUCH rather know when sitting down at the table that players X and Y often stab each other in the endgame than know that they are friends IRL.
In my case the mods decided (and I agree) that it didn't affect my play, I didn't know it was wrong, and I just couldn't play games with my RL friends anymore unless we all announced it at the table before the game started. (ie no punishment just a don't do it in the future) This seems to make sense in an odd way, and is how I interpret the rule now. If you know /anyone/ playing the game IRL you have to tell /everyone/ before play begins.
So it's fine, I'll do that if I ever play in a game with them again. At least that is clear...
But again, this seems an odd privileging of RL relationships (and what does that even mean? am I more/less likely to ally with them etc) when other relationships arguably have much MORE of an impact on how the game is played. ie; people arguing in the forums here, or observed diplo tendencies, or revenge for two stabs in a row, etc.
It would be nice to imagine every game happens in a vacuum. And for those that want that, I suppose anon games with enforced press styles are about as close as it gets. But otherwise, there is so many many things that effect decisionmaking - if not consciously than unconsciously - that picking one of them and leaning on it seems odd.
(Also for the record, I am the admin of a message board with over 10k members and I can assure you that moderators cannot read private messages. It is a bit strange. As someone who has access to the actual database files I could of course pull them out, but it seems off for mods to have access to PMs. And I think if google or facebook had access to emails without a subpoena then all shit would hit the fan)
My .02 as someone that hasn't been around long, was introduced to Diplo here basically through friends, avoids forum drama, and inadvertantly broke rules about playing with friends (I mean, aren't most games played with friends, that aren't played for money?)
Rule should be tell everyone at table if you know anyone in the game. But even that is weird and incomplete. And metagaming will always exist, at least unconsciously.