Well, sort of. In a broad sense an option is simply an irrevocable right to acquire or do something, without an obligation to actually acquire or do it. You can write options for whatever you'd like, despite securities being the most common use.

You're right about securities; the security usually exists prior to offering a contract on it. But even then it gets a little more complicated. For instance, I can write a naked option (an option to buy a security I don't even own). They're pretty flexible instruments, and I'm not sure how Safes are substantially different. That having been said, it sounds great for founders and funders, since it's far simpler than the alternatives, and I'm shocked no one has thought to do this before. Kudos to the YC folks for having the insight to set it up.