What in particular ... that particular person or IQ tests in general?
Both really.
An IQ test can tell you Einstein is smarter than than the person you meet on the street. But then you don’t need an IQ test to tell you that, it’s obvious.
An IQ test can’t tell you if Einstein is smarter than Newton or Edison.
There’s too much we don’t know about intelligence to make such measurements.
We won’t develop true AI until we understand much more about it (Intelligence).