Incandescent Fireflies not working as expected?

Upon promotion, one of the additional spells that the enchantress can use is the Incandescent Fireflies. Fully leveled up, this spell description reads “Deals 6% of target’s maximum life as Fire damage for each penalty applied to the target”. Originally I assumed that if I had 10 stacks of fire weakness, this would hit like a train and deal 60% total health damage as fire damage. But I think I answered my own question by writing this out. I guess this works by each unique penalty applied to the hero. So a fire weakness penalty and a burn penalty would allow for 12% max health damage as fire damage, regardless of the number of stacks of each individual penalty.

If I’m wrong, please correct me. That would make more sense as this would be ridiculously OP, which I originally thought it was. Would be good to use in conjunction with abilities/spells that apply multiple random penalties.