intense agony for ED does it work?

I see some build guides for PoE 2 who use intense agony support gem it says "50% more dmg with non ailment debuffs inflicted on full life monsters" but how is this gem usable at all? if you use contag on monster it already takes some of their life then you use ed and that gem is becomes useless no? what am i missing here

use ED and then Contag ;) More skillshotty, but it works.

so i need use ed then contag and then when it spreads the gem still works then i assume? cuz they spread to full hp enemies