Why does MM's lead reduce damage penalty when none is dealt?

When the ability used is not a damage dealing one (like Pao's T'challa or however it's spelled or Chirrut dispelling) then all assist but penalty is reduced
Wording is weird, I know

"Mephisto_style;d-238250" wrote:
Just curious what mechanic I am missing.
Call rebel fighters to assist dealing 90% less damage.
If no damage is dealt... "=0", reduce 90% penalty of 0 by 45%?
The dispel debuffs is cool, but what is up with that math function?

So, essentially, here's what's happening. Whenever a Rebel Fighter uses an ability thay call all other Rebel allies to assist, dealing 90% less damage. UNLESS, that ability did zero damage, then they still call all allies to assist, dealing 45% less damage. So essentially, this ability is triggered by Mon Mothma's basic, Field Command, and other Rebel Fighter abilities that deal no damage.

On a rebel's turn, they call up to a 5 assists (4 teammates, plus summoned rebel). If the rebel who is calling assists does no damage (MM basic, or maybe target had foresight), the assists suffer a 45% percent penalty, instead of the normal 90%.

I think you misunderstand, that if an assist call did no damage it's damage penalty was reduced. Which would make no sense, as you point out.

Note- I've defined rebels as Rebel Fighters + MM.