Actually Larian very likey actively wants to mislead the player in this regard.
After all how to fix Karlach shouldnt be easy and straightforward.
This statement is also not inconsistent with the character who speaks this line. People usually dont weight every word with gold. People making unprecise statements is very common.
I thought so too, that it was purposefully misleading.
Though the way things go with her, it just adds insult to injury.
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Regarding all others, I am not discussing her ending (there's the huge thread about that already). I just wanted to register a suggestion in regards to wording. The game felt very mindful of minorities and the portrayal of several types of struggles and I've read that Karlach's could be a reference to terminal illness. If so, I feel there is even more need to avoid the word cure.
However, if it's the writer actually trying to mislead the player into thinking that, well. What can I do. Felt there should be a bit of a follow through with that, in this case.