GRE Verbal - Kaplan Contradicting Verbal Grail (Weakener Questions)

[Char123]

 So, in the verbal grail book, it stated that the "evidence is a fact so it can never be questioned, thus the answer to a weakener question will never question/negate the evidence."
If you look at the attached file from a kaplan MST test, you will see that Kaplan contradicts Verbal Grail as Kaplan states that a correct response would either cite an alternative explanation OR DISCREDIT the EXPLANATION GIVEN IN THE PASSAGE. 

So now I am confused as to which one to follow. What do you guys think?

 

[PeakPerformance]

If the question tells you that you should consider a statement as true that contradicts the evidence in the passage, you should consider it as true.