I interpret the the last ruling in Gatherer ("you may choose either option") to say that you can still choose to sacrifice the Dreadnaught even if it is not on the battlefield.
I wonder how that fits with the rule that says you can't choose an impossible action. Isn't sacrificing something that's not on the battlefield impossible?
608.2d If an effect of a spell or ability offers any choices other than choices already made as part of casting the spell, activating the ability, or otherwise putting the spell or ability on the stack, the player announces these while applying the effect. The player can’t choose an option that’s illegal or impossible, [etc...]