Yeah, I think that is pretty sneaky (and a little snarky), too.
A lot of cruise lines, even Carnival and Royal Caribbean, will let you bring one bottle of wine on board and only charge a corkage fee if you ask them to open it in the dining room. But that is my understanding with Norwegian as well, that the corkage fee applies everywhere.
I have seen people bringing soda, water and more soda on an NCL cruise, no fees for that.
I have also seen people getting caught trying to sneak a full bottle of tequila in a suitcase - they were not smart enough to change the bottle.
If you really want to "sneak" alcohol onboard there are ways, but I am not sure it is worth the trouble.