I mean it's not that existing cleansers are valueless...Simplified the fight is Nilgal and two invincible watchers. The watchers gain some CR at the end of your units turn(? %) and additional whenever you use a non-attack skill(20%), they apply poisons when it's their turn. You do less damage when debuffed and Nilgal gains some CR if hit by a unit that's debuffed. Nilgal's ult strips buffs, applys unhealable, and gives a poison stack for each buff stripped.
So the watchers penalize fast units, most(all?) existing cleanse/immunity skills, and the fight is poisons galore so Lilias giving you a cleanse that doesn't proc the additional CR on top of other knight/kit benefits is absolutely strong for the fight. However the existing cleansers are still usable/ necessary, immunity is still strong; they're not sidelined.
But they're still punishing you for bringing them.
For bringing literally the entire extant cast of cleansers outside of Lilias.
No matter how I look at it, it's scummy as fuck. Even if Achates works (if you build her super speed with Magahara's) you're acting against their design intent which is to force you to bring someone like Lilias... when LIlias is the only character who can do what she does. Just on principle I really hate this style of design.