This is only when leader is alone.
When leader is attached to unit, use number of dices appropriate for the unit.
Example:
Elephant attacking Heavy Infantry with Leader will roll 5 dices, not 1.
PS. Why 1 die when attacking leader with elephant? My imagination tells me that slow Elephant attacking leader on horse (yup, in CCA all leaders are on horse) has minimal chance to actually get the leader... On the other hand, why MI uses 4d and HI 5d when attacking lone leader who is on horse?