wouldn't HUMS pick up problems before a possible engine problem became significant enough to cause control issues?
Yes, but only if it was a fault that had started to develop before the last IHUMS download.
I don't know how long this particular aircraft had been flying, but if it was on the return sector, it may have been 2-3 hours, during which time a sudden engine fault may have occured, so IHUMS wouldn't have helped.