Ninja means literally hiding your bets , there's a function in settings if you want to keep your bets/wager/stats hidden , and you'll appear in any leaderboard as "Ninja"... and pretty much every gambling site has this function.
Yes this "hidden feature" is available in almost all casinos because it is part of security and privacy. With this feature players are free from beggars and scam attempts that may try to steal the account by brut forcing password. The one in duckdice is unique because it is using different name "Ninja", while in most other casinos will be only something like "hidden".