[Infinite Wealth]Bro, why is the dungeon and NG+ seperate to a DLC?

Tried to 100 percent IW. Love the game and finished it. Tried to find new game plus and this Big Swell dungeon. Bruh i didnt know it required dlc.

Was this genuinely a feature they forgot and added last minute as paid dlc? Or was this one of those scummy publisher decisions where they cut the game into 100 pieces and you need to pay to get the full game.

I was honestly psyched to speedrun through NG+ Legend just to get achievements but damn now i gotta wait for sale just to get the dlc