Cydia has been updated. What thy meant by it not being official is that Saurik wasn't prepared for the release so he hadn't updated Cydia. They used official poorly. They just meant it hadn't been updated to ensure it works with iOS7. New versions of iOS can introduce unexpected functionality with a program. Cydia is no different in that respect.
Mobile Substrate is something of a backbone to jailbreaking. Most apps require MS. Conceptually, think of MS like a simulator. It predicts when apps will cause your phone to crash. Rather than letting it crash, it reboots to safe mode to allow you to eliminate problems rather than run into a brick. Without it being updated, many apps won't install. There are some MS work arounds right now. It's the holidays, so you'll need to be patient with Saurik's time as he updates it. The update is coming, just not here yet. With all the Taig drama around this jailbreak, it's safe to assume Apple will fix this before 7.1. If you want a jailbreak on 7.x any time soon, you'll want to update to 7 before the 7.1 drop. The jailbreak appears stable so far. But, there isn't much you can do to add functionality until a stable MS is released. This would be stable regardless of the jailbreak you use. It's just another package installed within Cydia.
Does all of that make sense?