The article says 100K, not 100M. I'm guessing that's what the parent comment meant.

100MLOC for an image format would be bananas. You could fit the entire codebases of a couple of modern operating systems, a handful of AAA videogames, and still have room for several web apps and command line utilities in 100MLOC.

the article includes test code and encoder code, that is not the way how we compute the decoder size

the decoder is something around 30 kloc