The grandparent appears to be assuming that all IE users have silverlight, when only 40% of them (about a quarter of all users) have it.
But you in turn forget that there is a similar Java plugin that does the same thing and is installed with Mac OS X by default. All combined I'd say 95% is a fair number so maybe the grandparent was aware of this option and simply didn't mention it explicitly.
Vorbis audio, which is playable in later versions of Flash, must be as close to 100% of desktops as you're ever going to get.
You're conflating HTML5 support and plug-in support. For instance: that Flash supports Vorbis does not mean that Flash-supporting clients can play <audio src="foo.oga">. It just means that the site can choose to deliver <audio> to one client and <embed> to another. The same situation happens with h.264.
Which goes to say: this is a good thing for Theora, but it is not a victory over h.264. Even in the most optimistic estimate, including this kind of plugin-as-equivalent conflation, it would be a tie, and only in the single domain of playback in desktop browsers.
