That is indeed true about that deck. I discovered it about 7 years ago. You have to climb 2 flights of outdoor stairs though. There is no elevator to get to that deck. Was last on the Celebrity Constellation about 4 years ago and the sign was no longer there. Pushed my lounger up into the front corner and sunbathed nude. One man showed up and wanted to take a picture to show his friends at home. I told him to get his wife up here and take her photo.
If you are daring, go for it. I would in a hearbeat. The most that could happen is security will tell you to put a bathing suit on. I don't think it is likely anything else would happen to you.