It find it amusing that when dress codes go unchanged for an extended period of time, they become requirements for period costumes. I don't know anyone my generation that would wear a collared shirt tucked into shorts, unless they were cosplaying as an 80's tourist dad, but at country clubs it's pretty common, to meet dress codes.
The man is such a prima donna. First the Niemann dispute, now he needs a dispensation because he won’t stick to the dress code rules.
From all we know, this was most likely an oversight by Carlsen and then an overreaction by FIDE. After he left on his own, I guess the main sponsor got pretty upset with FIDE and forced this dispensation to let all of them somehow save face and keep the star in the tournament.