Disclaimer: I work at Automattic but not on WordPress.com or Tumblr. This is all based on my impressions and personal opinion.

From the perspective of the user it’s just another blogging platform. Tumblr blogs are not self hosted and isn’t based on open source code at all. Tumblr code seems too specific and part of a bigger machine, so it also doesn’t make sense to make it open source. AFAIK goal is to make this mostly invisible to the users and their blogs will be powered by open source.

Any improvements to Tumblr blogs will have a chance to make it into the core of WordPress.

I’d imagine it’s also going to be way easier to move your blog to any other hosting provider, giving the user a wider choice.

It's described in the article that they're going to use Wordpress as a sort of "backend" platform for the existing Tumblr front end interface and that this is to mesh with their existing hosted architecture.