Simple answer-- it's a mistake.
In fact, I just checked 2nd edition rules, where it specifically says:
Flags rolled against a leader when alone in a hex, have no effect.
Yes, whether Close Combat or Ranged fire, you take your "normal" shot at a lone-leader and if you don't get a helmet, he has to evade.
So that's an error in the reference card.
Here's something to ponder that's the way the rules are written.
I'm close combating a unit with a leader, eliminate the unit and the leader evades...
When the attached leader’s unit is eliminated in Close Combat,
the attacking unit may Momentum Advance onto the vacated
hex after the leader Evades out of the hex.
But if I'm close combating a LONE leader...
A unit attacking a lone leader in Close Combat may not Momentum
Advance onto the vacated hex after the leader is eliminated
or Evades out of the hex.
There's a poser-- And it has been verified on the most recent FAQ.
Maybe the "shock" of seeing a leader alone in a hex just stuns the attackers so much they can't think about moving into the hex, whether they kill him or not.