I agree. Personally I think the definitions of cheating depend on the game itself and what you agree. On this site, I would say it is cheating to do something you couldn't do to another player. For example, if you're in a game with some people you know in real life, I would say that as well as metagaming/multiaccounting etc it would be cheating to go and look at their screen whilst they input their orders, because there is no way other opponents could do this to you. Thus with the limitations of the site, cheating over phpDiplomacy becomes impossible other than meta/multi's.
To answer you main question about the definition of cheating, I think that cheating takes two forms. First is the "real" cheating, which is things you can only do by exploiting the system and running different accounts. This would be impossible in the game, and anyone doing it is obviously doing it for the wrong reasons. Second is the "soft" cheating, which involves things like looking at people's orders, and can be considered in the same frame as trickery.
On this site arguable neither should occur, because either would be taking advantage of a situation that could not occur in favour of another player. In an face-to-face game however, as long as everyone agree's, you could allow cheating right up to the point of a Flying Dutchman