It seems that this will not be multiplayer-only after all. I don't know whether that was always the case and they just failed to communicate that, or they decided to include a singleplayer-component after people wrote to them about it.
In any case, I wrote them a message:
I will not back a multiplayer-only game, which is a pity in this instance, because had it not been multiplayer-only, I would definitely have backed this.
I don't like being dependant on other people to play a game. Apart from the fact that many people you run into online are idiots, multiplayer-only games can only be played as long as other people are playing them, which means they have a limited life span.
I know I am not alone in this. I'm not expecting you to change your plans (I also know that multiplayer-only games are significantly easier and hence cheaper to produce), but I think you should be made aware that you are deterring some people from backing your project, by not including a singleplayer component.
And here is their reply:
Thanks for your message.
Good news! An essential part of GODUS is single player. We all love to take a time to hone our godly powers before venturing into the exciting multiplayer crusades.
We hope you'll consider help us make GODUS a reality.
Take care,
22cans
The project also has a FAQ entry about this now. I don't know if it was there before.