IANAL, but to me it seems more like "we are actively defending our trademark (ala the hiring of the legal company), but we do not believe that you are infringing on that trademark and what you are doing is fair use, so carry on"
DannyBee (who is a lawyer) pointed out in the thread last night that this doesn't need to be "defended" against. He explained that, since they use the term "Tumblr" to refer specifically to the real company Tumblr, and since it's clear that they're not trying to masquerade as Tumblr or cause any confusion, that this is a legitimate use of the term and does not need to be "defended against" in order to protect the trademark.
This is correct. However, I think it is good practice to properly attribute trademarks in those cases to make sure that it is clear that it is intended in this way.
A small disclaimer at the bottom of the page saying:
Tumblr(R) is a registered trademark of Tumblr, Inc. which is not affiliated with this site. All other trademarks are owned by their respective owners.
goes a long ways towards avoiding problems like this.
