It'd be better to think of it as "Last week in the WebKit project". As as the post points out, the WebKit nightly is only representative of the mac port of WebKit, which Apple mostly calls the shots on. In many cases Apple takes a conservative approach when enabling features that other ports may be implementing or experimenting with.

Peter's posts talk about everything that's going on in the WebKit project. The nightlies are just builds of Apple's Mac OS X and Windows ports.