I think the exact code for features in that diagram does fit in 5K lines of code. More specifically an entropy coder is less than 1K LoC, the modular mode and transform is probably 1.5K LoC, the entirety of VarDCT is probably 2K LoC. The problem is that JPEG XL has a lot of other features to make it viable as a long-term format, and I greatly underestimated that complexity at the beginning.