I've been waiting for an answer to this question for a few days now:
https://devzone.nordicsemi.com/f/nordic-q-a/37487/why-is-the-hfclk-necessary-for-the-dtm-module
Is there a standard way on the DevZone to politely ask "hey, can someone look at this again?" I accidentally clicked "Verify Answer" at one point (but undid that), so that may have caused some confusion.