They could still work on assets and just tag them into the engine. You don't need the engine before you can start developing for it. Mount and blade 2, aka Bannerlord has people already developing mods for the game, and the game isn't even in a playable phase without commands. They could probably get done with key assets like models, scenery, textures, scripts, and sounds before source engine 2 even comes out. That leaves the game about 74% done. What's left Is rigging, animating, and waiting for lighting and lightmaps to render.
