The author says the person is paid "a reasonable contractor fee for the work", which I would hope is equal to or greater than the current salary. If not, it really doesn't make sense, as you say.

I would be curious to know what happens before this 5 day test. Do people just walk in with resume in hand and sit down to work for 5 days? Probably not. There would have to be some sort of discussions-that-aren't-an-interview which would lead to a yea-nay decision on the work term. What sort of funnel is there?

I agree there is a big commitment on both sides to do this 1-2 week work stage. There has to be more to the events leading up to the stage than what's in the article.