According to the article they actually tested the first assumption and it was true.
The second assumption is not required. If people who score a 95 are only 5% better at the job than people who score a 70, all else equal you'd still pick the person who scored a 95 given the choice.
Non-linear doesn't mean "still monotonic". My experience has been that beyond a certain threshold on a given test, job performance is essentially uncorrelated with test performance.
As for the article, it's not given me particular solid vibes, a feeling not helped by some of the comments here (both pro and con).
Satisfying the first assumption means "still monotonic".
Also, if you had a better test then you'd use it, but at some point you have 10 candidates and 5 slots and have to use something to choose, so you use the closest approximation available until you can come up with a better one.
> Satisfying the first assumption means "still monotonic".
Sorry, but I just don't agree. There are "qualifying tests" for jobs that I've done that just do not have any sort of monotonic relationship with job performance. I'm a firefighter (volunteer) - to become operational you need to be certified as either FF I or FF II, but neither of those provide anything more than a "yes, this person can learn the basic stuff required to do this". The question of how good a firefighter someone will be is almost orthogonal to their performance on the certification exams. Someone who gets 95% on their IFSAC FF II exam is in no way predicted to be a better firefighter than someone who got 78%.
The second assumption is not required. If people who score a 95 are only 5% better at the job than people who score a 70, all else equal you'd still pick the person who scored a 95 given the choice.