"If a player concedes in "Assassin" the last person who dealt damage to him will receive the 2 points. However, if it is reasonable that the next player could have dealt damage or killed that player, then he/she will receive those 2 points instead."
Thanks for the confusing PM, but this doesn't answer the question.
Is conceding legal or not?
And if your PM is an official "RULE", it should be posted in the rules section, along with a sentence that says "Conceding is legal".