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The plane of focus must be reflected on the ground showing a narrow sharp line that is absent on the shot. Moreover the foreground is heavily patched and has some evident areas of recurring textures/patterns. And it's a little bit more than "blah,blah, blah... just levels honestly". No doubt he is lying. Though I don't have enough facts to state that the background picture and the boy shot were taken separately.
If you read the thread to the end, he added that he had forgotten that he extended the foreground via cloning. This is why the ground looks so weird. There was originally too little foreground under the kid's feet.
I am not sure that it is fake beyond that. The depth of field is a bit odd, but so is the dof of the 50MP, and I don't know this lens. I don't like the look particularly though. As Samuli says, it looks like a cardboard person in front of a blurry backdrop, nothing inspiring there, and the colours look fake or wrong. I have never been that impressed with the photos I have seen from the 85L, but I am careful about saying that, since it has so many fans.