It's not at all clear from the mere existence of a link at the bottom of the page, which isn't even put there by SPC itself, that it is a contract and that you are assenting to it by creating an account.
A similar case involved Netscape, which had a license agreement for a download manager called SmartDownload. However, despite the existence of the agreement, you didn't have to indicate you were actually agreeing to it. The court's reasoning:
Specht v. Netscape, 306 F.3d 17, 29-30 (2002). The procedural history here is a bit ugly, in that it's a case in the Second Circuit which is appeal from the Southern District of New York, concerning an attempt by Netscape to enforce an arbitration agreement according to California law, because of a choice-of-law provision.
Still, the question is whether someone agrees to a contract by using a website that doesn't ever explicitly tell them what they're agreeing to, and even the admin team seemed confused in chat as to what the contract actually is and what it means. I don't actually know the signup procedure for SCP except it requires you to sign up first to wikidot, and then to SCP. It seemed rather arcane when I looked at it years ago. I don't remember any explicit clickthrough or even directions that such a document existed, but I probably wouldn't after this long.
In any event, it is fairly obvious that to agree to a contract, you have to have at least been on notice of its existence. Barring unconscionability, though, if you do have a clickthrough of some sort, where you must click a button saying something like "Agree," such that you should be aware you're actually doing so, you've agreed to it even if (like the vast majority of people) you completely blow off even reading it.
Do we know for a fact that there is no such clickthrough or other prominent notification such that a person signing up could not do so without being put on notice and actually agreeing to it?