Why would smart people care about denim vs. trousers?
Let competitors wear pajamas- it makes no difference.
Mostly it's about the sponsors. It's much more difficult to get sponsors for an event if the participants are dressed like they slept in their clothes. That's why organizers try to impose minimal standards on dresscodes.
Jeans and sneakers are maybe debatable, but players showed up with cargo pants, shorts or tank tops on other events.
In the FIDE regulation for that event jeans were explicitly mentioned as not allowed. FIDE would have made a fool out of themselves when allowing Magnus to wear the jeans.