As far as I recall (and it might not be totally correct), there was at least some indication or implication from mods that it was legitimate. After the fact, I think Sicy or someone posted that they had done some diligence on it and felt it to be verifiable.
So, that was after the fact of course, which begs the question how did rational people actually believe it before all that came out..well..I know for me I was relatively confident that the mods would never knowingly allow an imposter to play that cruel of a game on the board, and the very fact that it was allowed to continue seemed to indicate to me that they must have done some sort of check and believed it to be true. I had alot of faith that the people who run the forum would be all over that kind of thing, being huge U2 fans themselves. Is/was there also a rule about not being allowed to call yourself "Bono", "The Edge", "Adam Clayton", or "Larry Mullen Jr"? I think there was/is. In other words, for a person to have even been allowed to set up the account and start posting as "The Edge"..it had to be real. That was just my belief on it, can't speak for others.