I flat out don't take arxiv papers seriously without a lot of corroboration. Basically ~anyone can post ~anything to it. They do spam moderation but no content review.

Also that paper admits the problem turned out to be pretty trivial and was only unsolved because nobody had bothered to try that hard (page 11)

There's a lot of problems with paid scientific journals being a walled garden and I am by no means defending that system, buuuut it's also true that anything published to an open repository is almost certainly there because it wasn't good enough for anything else.