Ok. I figured the acid type as poisonous/corrosive was implied. I was wrong.
Sulfuric acid is naturally occurring and is the main acid in acid rain, though that wouldn't drop a puddle below PH 6.
The problem is PH dropping to 4 (similar to vinegar) where stuff cannot live in it - you cannot drink vinegar as a substitute for water and swimming in it is going to be caustic for eyes, nose and sensitive areas.
Since everything in the pond is already dead, they can dump some potassium hydroxide to neutralize most of the remaining sulphuric acid, then flush it out by diluting it down to a level that won't cause any more damage downstream.
The likely more problematic bits are whatever else might be in the chemical spillage: the sulphuric acid PH shock may have killed everything but it will sort itself out in due time if you simply dilute it enough while some toxic chemicals such as Dupont's C8 are considered "forever chemicals" because they have no safe concentration, don't break down naturally and are seemingly impossible to get rid of using practical methods.