Some conferences require that you publish your code. So all they have to do is add to the paper "our code will be published at this url: ..." and then once they

Some conferences require that you publish your code. So all they have to do is add to the paper "our code will be published at this url: ..." and then once they're accepted they never do it. Similarly supplementary material makes their paper seem better and more solid than it is, so they say they have it then never publish it. It's just generally scummy behaviour from people who are under a lot of pressure to publish.