I don’t know why there was disagreement over this. Overdose, in common parlance, indicates too much of something. This was not too much, this was an illegal substance specifically created by mixing two things together. They didn’t take too much of it, it was a deadly mixture.
Sorta like ammonia and bleach. Sure you'll have clean floors, but you're imprecating your lungs and eyes to the caustic fumes of chloramine.
If they are dying, then they took too much. They took too much because they were not expecting a much stronger drug to be mixed in with their heroin.
It’s a semantical thing. If I normally inject half a syringe and I injected half a syringe today, I have in one sense given myself the same quantity. So it is not a change in dose.
But if the substance injected is stronger, then I have taken more of the “active ingredient” and have overdosed in that sense.
If I normally drink a gallon of beer and instead drink a gallon of wine, I have taken the same quantity, but ingested more of the active ingredient. If I do this unwittingly, I could drink too much.