It's important to define was "equal" is in this context. Some people hear "equal" and think they must measure exactly the same in every test, but that's not how the word is being used in this context. It's more that people are so varied from one person to another that no test can truly judge them well enough to differentiate them when it comes to inherent worth.

One person might measure above another in one test, but there are surely many others where the results would be flipped. There are so many different things you could test a person on that in the end none of them really matter; any one measurement is like trying to figure out what an extinct animal looked like from a single tiny piece of a fossil.

That's what the IQ test is doing - it's taking one tiny piece of human intelligence, which itself is one tiny piece of what might be said to make up a person's value, and trying to use that to extrapolate information about them that simply can't be taken from such a 1-dimensional test. It's not worthless, but it needs to be paired with a bunch of other tests before it can really say anything, and even then it wouldn't say much.