I'm not a genius about graphics, but those pre-rendered backgrounds are basically pictures which the characters are then placed on. Although I suppose they must have some sort of texture or model when they were initially rendered. But I doubt that they can just be picked up & re-used.
But I think what Squenix is trying to say is that they don't want to find short-cuts to make VII. Yes, there are a whole lot of things they could just pick up from VII. And they could have a World Map lacking detail. But they want VII to have the production values that they achieved with XIII.
However, to say it's going to take 10 times as long because of graphics is a load of rubbish. Elder Scrolls IV took 4 years to make and it makes VII's entire world look tiny. It has much more dialogue (all fully voiced).
I have a strange feeling that a VII remake might not be that far away actually. Why else would Square-Enix be making statements like this that make no sense?