This is some strange misinformation.

The C++ JPEG XL decoder is ~30'000 lines, i.e., 3000x smaller than you claim. A non-multithreaded, non-simdified code would be much simpler, around 8000 to 10000 lines of code.

It is not difficult to measure from the repository. The compiled compressed binary for an APK is 5x smaller than that of full AVIF. The complete specification at under 100 pages is ~13x more compact than that of full AVIF.