Melanson basically just repeats himself several times that the Flash Player program is far different from normal Linux video applications that are solely tasked with playing back videos. These multimedia programs already support VA-API, VDPAU, XvBA, X-Video, and XvMC, but Flash Player is more complex so he prefers to pout. The Flash Player has more to handle since first it needs to convert all of its YUV data to RGB color-space and then it also needs to read the decoded video in order to display it within the web-browser window.
In the end Melanson says that none of the Linux video APIs are implemented yet as the available APIs don't allow the programs to easily read the decoded video frames. The Windows video APIs do, but Adobe has more than enough partners at Intel, ATI/AMD, and NVIDIA to get this changed. Hell, they could even provide the patches to extend the support themselves (for VA-API at least) in this open-source world. However, the Gnash player plug-in that is a free software implementation of Flash/SWF on Linux, has already a VA-API patch.
Additionally, Melanson claims the Linux Flash Player doesn't support the Broadcom Crystal HD co-processor yet (but the Windows Flash Player does) as the Linux drivers are not ready. The Crystal HD Linux drivers are open-source and ready and the XBMC project has already implemented 1080p video decoding support using this Broadcom hardware on an open-source stack.
Some of Melanson's claims are justified, but in the end Adobe can be doing much more -- without investing too much -- to improving their Flash Player support on Linux. Update: Gwenole Beauchesne of Splitted Desktop Systems has commented in our forums that Adobe's claims regarding the inability to access decoded frames in the available video APIs are invalid.