Note that he proposes the audition project to be "a regular consulting gig with an hourly rate", spent on-site doing real work.
You don't have time to earn money doing real work? Then why are you applying?
And even for a "truly good" professional who has his pick of a dozen offers, I could see this as a good idea, since they have just as much reason to be interested in the company's "real" work environment than the company has to be interested in their "real" work performance.