Apple only updated the Flash Plugin via Safari updates.

Mac OS X software and security updates have updated the Flash plug-in. Safari updates have never updated the Adobe Flash plug-in.

This is simply untrue.

Yes, WebKit still supports Tiger, though some of the newer WebKit features (hardware-accelerated compositing, 3D transforms, etc) aren’t available as they require support from the underlying OS that isn’t present on Tiger.

That's not an accurate way of looking at it. WebKit has no support for video codecs, period. WebKit delegates media decoding to the underlying platform, much like it does with image decoding. The WebKit tree (at http://svn.webkit.org/) supports three different media backends: QuickTime for Mac and Windows, GStreamer for GTK+, and Phonon for Qt. Both QuickTime and GStreamer have pluggable codec support, and WebKit-using applications will load any video that it has codec support for.

What the hell do you think WebKit will use to be built? LLVM.

Err, what? WebKit is compiled with GCC on Mac OS X (4.0 on Tiger, 4.2 on Leopard), and Visual C++ on Windows. Compiling WebKit with llvm-gcc is possible, but I'd only recommend it if you want something that's slower and takes longer to compile.

It's likely that the grandparent is referring to the SquirrelFish Extreme (AKA Nitro) JavaScript engine developed by Apple for WebKit.

As a developer working on WebKit, this is completely wrong and more than a little insulting.

The versions of WebKit included with Safari releases are built directly from the public tree. There is no secret version of WebKit that Apple fixes bugs in for Safari releases before eventually landing the changes in the WebKit tree. The WebKit tree is Apple's official WebKit tree, and is where all of Apple's development on WebKit for Mac OS X and Windows takes place.

For sake of reference http://trac.webkit.org/browser/releases/Apple/Safari%204%20Public%20Beta contains the exact source code of WebKit that was built and released as Safari 4 Public Beta earlier today. There are no secret changes in the version of WebKit that Apple shipped. The changes are all there in the open for the world to see.