At the time, the game was using the old Build Engine.
Leaks of early builds of older games continue: following the 1995 Prey, players received the 1996 Duke Nukem Forever build. The game was not released until 15 years later, in 2011.
Over the years of development, Duke Nukem Forever has gone through many iterations. The 1996 version was built on the old Duke Nukem 3D Build Engine.
A year later, the project was restarted: the game was ported to the Quake 2 engine. Another year later, the game was relaunched, followed by legal conflicts, and the project was effectively frozen.
Apparently, one of the earliest Duke Nukem Forever bills has been made public. Notably, in 2022, Duke Nukem Forever fans received an early build and another version of the game from 2001, made on the Unreal Engine.