Correct me if I'm wrong, but it looks like other browsers are free to implement the "pepper" API as well. It seems as though they are not saying "we're only supporting Chrome". They are saying "Chrome is the only browser that has implemented Pepper so far, and we're only supporting Pepper on Linux".
Think licensing. If the Adobe blog post is correct and the pepper plugin is only distributed with Chrome and isn't open source and part of Chromium then how is another browser going to support it? They'd have to tell you to install Chrome and then open the plugin from within Chrome's installed directory. Pretty ugly solution. If it's open source and part of Chromium then they can at least take the source out and ship…
Meanwhile, Flash continues to circle the drain, and this might hasten its inevitable, but slow demise.