And as I originally compared it, you can also solo all the time in WoW and in effect play it as a singleplayer game, but it still isn't that. D3 was designed as multiplayer only, the developers even said so when the internet thing came out. I believe the quote was something like "playing this by yourself is playing it wrong."
Sure, and why your analogy is wrong has already been pointed out to you repeatedly. On the off-chance you'll actually read it this time, I'll point it out again:
You can try
and play WoW solo, but you can't actually do it. You're in an open, persistent world, with other people running around impacting the world. Sure, you can avoid interacting with them, but they are interacting with the same world you are, meaning that even with avoidance, you are still in a multiplayer environment.
The same is not true of D3. If you make a private game, it's private. No other player can join you. You aren't sharing a world with anyone else. It is, in all sense of the word, a purely singleplayer experience.
As for developers saying things, I'd simply point out that the developers of Mass Effect 3 told us how the ending was going to be based on the choices that players had made in the game. In other words, developers can be wrong.