The only way they would have inside info would be through corporate espionage. Apple certainly isn't giving them anything. They are only basing these representations (that's all they are at this point) on the supposed design documents that were allegedly leaked last month (when they were leaked, you may remember that MR posted a rendering of the possible appearance of the iPhone 5). I'm sure they have done a few test runs based on those documents, and are geared up to go if those specifications prove to be bona fide, but I'm sure they have not already stockpiled thousands of these cases, ready to be shipped the moment Apple finally unveils the new iPhone. It's still just speculation. Expensive, high-stakes speculation, to be sure, but for them it's a gamble worth taking if they can corner the market on iPhone cases for the first month or so.
You have no idea where Case-Mate is getting their info. To suggest they are basing anything off those rendered drawings is hilarious.
Case-Mate already knows the dimensions of the next iPhone. The cases they leaked are going to fit the next iPhone. I'll eat my words if I'm wrong, but I'm not.