It's bullshit. Think about how complex the human brain is, how some people are able to do shit like multiply huge numbers instantly, have photographic memory, then you have stuff like synesthesia, hallucinations, etc. Basically, it is uncontroversial that the brain is both very very complex and still largely a mystery to us.
Now, do you think it is reasonable to think that such a complexity can be represented by a single number? It's absurd. It's akin to saying we can come up with a numerical, hierarchical classification of planets with Earth scoring a 100, and then others scoring below or above that depending on various qualities. But what would it mean if Earth is 100 and Mars is 35, and Mercury is 10? Absolutely nothing, because lots of information is lost. Is it atmosphere, soil quality, presence of water, of organic material...? We obviously can't test every single aspect of a planet, because it is too much to do and we are unable to do it. IQ was designed in the same way, European (white) males are taken to be the standard against which others are tested. Europeans designed the test, and Europeans are best at taking it.
But we don't even have to go that far. We can use the IQ-ist framework to ask how can someone of presumably average (or even above-average) intelligence invent a test that can test someone who is far more intelligent than the test maker? In other words, the intelligence test still has to be devised by a human, and the human who devises it knows all the answers to the questions. Therefore, the person who invented the test cannot ask questions that they themselves can't solve, while someone of a superior intelligence may be able to.