Umm yes it is?

"Some economy of data is possible where a frame need only rewrite a portion of the pixels of the display, because the Image Descriptor can define a smaller rectangle to be rescanned instead of the whole image"

Every frame of animated gif can choose to modify small portion of previously drawn image. This is why you cant display animated gif starting in the middle, you will only render moving parts until you loop whole thing.

This is precisely what Author implemented, he is encoding changes between frames, this way cpu has to only modify parts of display memory that are changing.

How is that not the way animated gif works?

This is delta coding; it's part of animated GIF, which then applies LZW compression to the deltas. This guy has used RLE compression (like TIFF, I believe) instead.