The project was in the beginning. It was made by 18yo people in their free time. Of course they couln`t reach the quality of a game made by 100 professionals in years of full time job. But: It`s impressive what they nevertheless did. The artwork fits together and the game has a unique style and was an eye catcher in the hobby programmer world back then. I`m pretty sure that it would have had the potential to look even better if they continued working on it. Thinking about shaders, lighting and level design aspects.
You`re a shader programmer, I`m mainly a level and art designer + scripting. And from my perspective I say: It`s also a question of lighting, shadows and color adjustments to let something look good or bad. You can have the nicest level of the world. When you`re lighting it wrong, it will look bad. Then, in second case, there`re some special effects like Bloom, normal mapping, other shaders. Look at the second SD screen I just added in the post with the screenshots. It`s just the lighting.

I know shaders are your special case, that´s why your view is focussed on that aspect. And they only had basic ones from today´s perspective. But that´s quite natural. Back in 2005 the A6 engine itself and shader support was brand new. The fact, that they understood how to handle this as an unexperienced, young team back then, is respectable.