Earlier quoted context omitted.
Text would have been redundant. The entire thing is launched. The interface is self-explanatory. (If you've never used Tumblr, it really is beautifully simple, despite this new clutter.) The video is there to get you excited, not to tour the site. Lines like "The first thing you'll notice about the new Tumblr is that it is sexy as hell " don't guide, they boast. Tumblr's staff blog is often very much about the people…
Text would not have been redundant: it would have allowed users to see everything that was new instead of having to crawl over the site looking for new stuff. Eg: I have to check three or four possible locations to see if I can now see who my friends are reading, instead of checking a list of new features. The evidence that text wouldn't have been redundant is even on this thread -- poster asks what's new, is answere…
Tumblr's video announcement wasn't the release. The release was the release. The idea's that you'll notice what's new while you're using the service. It shouldn't be a scavenger hunt. Most users don't work like that.
The video was just a fun little announcement because David and Marco like flashy things.