Not as drastic as the headline makes it out to be, or at least so they claim.
“We acquired Tumblr to benefit from its differences and strengths, not to water it down. We love Tumblr’s streamlined posting experience and its current product direction,” the post explained. “We’re not changing that. We’re talking about running Tumblr’s backend on WordPress. You won’t even notice a difference from the outside,” it noted.
We’ll see how that actually works out. Tumblr’s backend has always seemed rather… makeshift, so I’m curious to see how they manage to do that. Given Tumblr’s technical eccentricities, a backend migration could probably do a lot of good for the functionality of the site, if done properly. I have my doubts that WordPress’ engineers will be given the time and resources to do a full overhaul/refactor though, so I’m fully expecting even more janky, barely functional code stapling the two systems together.