Fair question. I've read a couple of critiques of Diátaxis that suggest additional boxes.

FAQs and "Gotchas" need a "home box". So do gists. Maybe a third dimension would be something like "degree of curation" or "need for taxonomy and organisation" ?

FWIW... DITA has concepts, tasks, & references - and AFAICT these map fairly nicely to Diátaxis.