I hope that isn't true. Perhaps I'm an aberration in that I never once played any of the Diablo's as multiplayer, only single player, but a three hour game?
The beta only has Act I in it. Not sure how many acts it'll have overall, but Diablo II had 4, with a 5th added in the expansion. Plus there are multiple difficulty levels, so you generally go back and re-do the content on a higher difficulty.
Which does bring up the following quote from the article:
He informed me that, based on his many hours with the first two games, Diablo never gets very difficult.
Total BS. The ease of running around in Act I Normal compared to running around in Act V Hell is considerable. Especially if you aren't dealing with a fully twinked out character.