(By the way, to avoid confusion: this is not a G5, it's a Mac Pro. The G5 for example didn't have four hard drive sleds in a row.)
Now I've done a hackintosh G5 hackintosh build, what was my first PC build by the way, I know a little bit more about the subject. This build looks so clean because the cables are hidden behind plates of aluminum (bottom and 'back wall'). I still think it's very well done, don't get me wrong, but in 2018 I thought this would be possible with just clever part picking and clever routing of cables.
Also, this build was an eye-opener for me in the sense that it proves that besides the fans in the GPU-card – that is not pointed outwards – and the fan on the PSU – that is in a different compartment – the BeQuiet! CPU-cooler add-on fan is enough. There are no case fans necessary!