Is the tweet depicting a flowchart of LZW decompression? ( https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Wel... )

It appears so! Having re-written the GIF implementation for Pharo and Squeak, I can tell you that it gets a little more complicated due to dynamic bit sizes of codes and things like that. LZW, which was meant for strings, is cleverly applied to pixels in the GIF spec