I hate to be nitpicky; but that’s a decompilation, not a demake.
‘Demake’ usually refers to a game that gets remade for a system older (or less powerful) than the one it was released for. A good current example is the in-progress Super Mario 64 demake for GBA.
‘Decompilation’ is where one reverse-engineers a game (or any software!) back to its original source code, or close enough that when you build it, it’s identical to an original copy. So, the goal of the Lego Island demake is to produce source code that can be built into a fully binary-compatible copy of Lego Island, indistinct from what’s on the original CD.