I don't even take this as a shot against Harden or KD by Giannis. I think what the intent of this, was that Giannis only cares about playing basketball, and then doing anything that is required of him. He won't go the extra mile in doing commercials, or trying to be the face of the league.
For him it's unneeded attention and a severe distraction from his work, and it takes away from it. Plus it would mean having more media coverage, so going to New York or LA. Giannis has always made it clear that he likes Milwaukee, because the city itself wouldn't try to hound him for a autograph; it would let him relax with his family, eat in piece, and maybe allow for a few photographs.