Tumblr lets you change subdomains easily. If Pitchfork chooses to relinquish the subdomain, the original owner could take it back immediately, regardless of how Tumblr Support acts.
If you've posted five times in a year, and the last one sounds like you've decided your blog is obsolete, then it's not an instance of somebody abandoning a longrunning thing. It looks like somebody gave the service a few tries and gave up.
Tumblr has a very informal attitude regarding staff blogs. Meaghan was a blogger before she ever worked for Tumblr. When she blogs about things, it's not an official response. It's merely her personal reaction. The staff blog is at http://staff.tumblr.com.
Tumblr employees wouldn't be using the service effectively if they were all stodgy officials. The fact that they're allowed to be themselves, flaws and all, is a feature.