The content types in <embed> or <object> tags and the modules that handle them are not specified in the HTML language (by definition), and are thus optional. HTML5 video IS specified, and is thus not optional.
"It's looking almost certain that users will have to download their own codec "plugins" (like MS's Chrome H.264 support pack) , so maybe HTML5 video really doesn't mean a thing."
Correct on both points, and that's the problem. Like anything else in the HTML5 spec, or any other Internet spec for that matter, it just has to work; otherwise it's worse than useless. In this case, it will only work if external, unspecified requirements are met (those being the choice of a codec), and that is a very serious flaw in the specification. Browser publishers cannot guarantee that their product will render all video because they cannot include the necessary codecs with their product, or even guarantee that they will be available at all.
That is why, as things stand, a browser's technical compliance with the spec is of little meaning in and of itself.
The availability of the WebM codec does not correct the omission in the spec; but it at least makes it possible to create full-quality video content that all browsers can render, and in the process respect the central design philosophy of an Internet that is open and free. It is imperfect--WebM is patented, but the patent-holder has expressly made it freely available, guaranteed, which is at least the next best thing.