Prerequisites

Those look like build prerequisites. Many decomp projects do not need original game assets at build time, just runtime.

and after that…

cdpath and diskpath registry keys […] point to the correct location for the asset files

I read this as another implication that original game files are required. Otherwise, why would you need a registry key telling the new game engine where to look for assets? The /assets file in the git repo contains only 3 pngs of icon images. There’s no way they’ve secretly bundled a whole game’s worth of models and textures in the codebase.

Implication from the readme is that the lego island decomp does indeed need original assets:

The simplest way to use the recompiled binaries is to swap the original executables (ISLE.EXE, LEGO1.DLL, and CONFIG.EXE) in LEGO Island’s installation directory for the ones that you’ve built from this source code.