I mean, they mental screened Rock Hound, and he bullshitted his way through it. Solving a rubiks cube is actually relatively simple, once you know the trick.
Rock Hound went to poo when things went serious on the big rock.
Maybe the psychologists were to blame? Like they tried to psych Harry up for the mission and convinced him to focus on his daughter's happiness.
But nah, Pride would have probably dragged Harry onto that drill. Much like Mordun Solus, "Had To Be Me. Someone Else Might Have Gotten It Wrong."