Mr. Khan is not saying that IQ does not exist or that it isn't important or useful. He is saying that to 'praise' a child for something innate is not simply unhelpful, it is actually harmful. Praising them for effort on the other hand does not at all 'lower' their IQ, but builds up self discipline and will power which when combined with whatever IQ they have, will generate far better results.
I know this first hand. I was born with a genius IQ. I taught myself to read at the age of four. It took me two weeks to learn enough to read comic books out loud to my little brother while he looked at the pictures. Everyone around me praised me for how smart I was. I was never praised for making an effort.
The end result is while it has always been really easy for me to learn stuff, to understand things, the hardest thing for me to do is to make an effort.
I did nothing to deserve being praised for my IQ. I did nothing to earn my IQ. My IQ was an accident of birth, as much as my hair colour. Should I be praised for being blond?
Making an effort on the other hand is something people do, it is something that can be learned, it is something that is praiseworthy. By making that the source of a person's validation, you can help a person evolve into someone who has the ability to make effort. Praising a person's IQ however gains them nothing other than to potentially give them a false sense of ego.