Well seeing as there is no such thing as cabin tips in the first place, I doubt it is legit. Most likely something someone typed up based on opinions and other people's post across the internet. There is a DSC (Daily Service Charge) that, according to NCL is used to fund employee incentive programs. How that all is handled, no one outside ncl knows and honestly it is not anyone else's business. Enjoy your cruise!
BTW per the FAQ on ncl's web site, while it is encouraged, tipping is not at all required on ncl.