That's not immediately clear from the verbiage (both places about the promise only mention "content you publish"), so that might be something worth adding.
He actually addressed that. He said its hard to promise to keep that stuff forever, implying that limiting the promise only to published content was an intentional decision in order to make it feasible to make the promise at all. Notice also that he did mention that they save that stuff right now, despite not promising. Better to promise to save what you can than to not save anything at all. I thought his response was quite adequate.