Vlad Like @pteroknob mentioned I think one of the big advantages of the Blink implementation is its selective inversion. Using the script you linked causes issues for sites like Amazon for example that have a dark navbar and footer which just become white since they're mindlessly inverted. There are also issues with some colors just becoming different colors because of this.
I also don't see why a script wouldn't be able to achieve this compared to something built into browser engine itself besides speed. I looked around and couldn't really find any scripts that accomplish this nearly as well as the Chromium option.
When I was looking at the Chromium/Blink source I saw some of the code for this but a lot of it's spread out between so many different files it gave me a headache trying to make sense of it.