"Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away." -- Antoine de Saint-Exupery

I guess getting the code down to 100 lines was just a side effect. They first had to find out what they wanted to achieve in the first place --- including all the nitty-gritty details.

After a while they probably understood their domain. The number of lines just shows this understanding.

Seems worth it considering the hundreds of millions of times that code has been executed and hacked on.

Sure, write your YAGNI version in five minutes, but don't expect anyone to help you. When you put effort into your public code, others will too.