Why say source code is available with an ArXiv paper when it is not?
There is no advantage to claiming the source code is available and then not doing that. All it will achieve is irritating the really interested readers.
The most likely reason is it’s a simple mistake. Something like they wanted to do that but forgot. Or other reasons, maybe department policies came up that prevented the author from doing that. Or maybe the author found another more suitable place to publish the code and forgot to update the link in the paper. Or...
The simplest solution is to contact the author directly.