They played fine on my laptop, but would not scan as gifs in iOS. It was a nightmare of a problem to debug - what are your search terms here?
The GIF specification, GIFs that work and those that don't, and a hex editor.
Given all the other impressive work in this article, I'm surprised that wasn't an easy problem for them to debug --- the latter seems almost trivial to me (find the differences) in contrast to the amount of creative thinking required to figure out how to run Doom in iPhone Photos.