The phrase “written up” here means that he had already done the core part of his thesis work two years ago, even before starting on this problem: he had already

The phrase “written up” here means that he had already done the core part of his thesis work two years ago, even before starting on this problem: he had already proved enough results, possibly major parts of which had already even been published as papers, and it only remained to put them together in the coherent form of a thesis, schedule a thesis defence, etc. But this writing work was interrupted by the challenge of computing d(9), and he's describing starting/resuming the work of “just writing” as a break, from the harder work he has been doing on d(9).