Correct me if I am wrong, but it seems to me that there could be a distinction for facedown spells that have not been cast. Since there is no mechanism in the game that forces you to reveal what spells you have prepared, whether they are on your person, a familiar, or a spawnpoint; I fail to see how it is relevant if they could actually be cast as long as they aren't illegally cast. In all the text I've seen about spawnpoints and familiars it only ever mentions restrictions on what type of spells it can cast, never a restriction on what kind of spells it can prepare.
It seemed pretty clear to me that this was the intention of the word Bluff. I don't see why they would have to mention it if it was only a matter of mana or legal targets, there is nothing in the rules as far as I know that would lead one to one to believe that they would be in violation of the rules if they weren't able to afford a spell they had selected or if there didn't happen to be a target for it.
If this wasn't the intent of that mention, I would like an explanation as to why it should matter or be specifically ruled against and also request that the rules be clarified to clearly state that a familiar or spawnpoint cannot prepare a spell that it is restricted from casting.