That's a silly argument. There are many jobs at which being smart is an advantage. IQ tests correlate fairly well with being smart. Giving an IQ test could then give you a group of people that are more likely to be smart than the initial interviewing population. Assuming the job is not "taking IQ tests" is the assessment unjustified?
As much as I hate the current interview process, this isn't true.
The justification foe the assessment is that the assessment is the defined set of hoops that the company who you are trying to get paid by has assigned.