Have just returned last weekend from an 11 night cruise on Celebrity Constellation.
Though they do say no jeans in their suggested dress codes, they weren't enforcing it. Walked through the dining room on a night designated Informal, I saw a table in t-shirts and wearing their ball caps in the dining room.
Not that this was appropriate... but it was fact, and no one asked them to remove the headgear, or a change of clothing.
If they were just in jeans and a t-shirt there's always the possibility that their luggage was lost. However, no good explanation for allowing the ball caps.