Guys who wrote the paper: "But it's substantial!"
Like... obviously the guys who wrote the paper think it's substantial, or they wouldn't have submitted it. The very idea of "we think it's good, so we want to publish this without anyone telling us it's not" is... silly. As a developer, this is like someone opening a PR, then getting mad if the PR is rejected upon a quick code review because "Well, I think it's good code!" Of course you do, that's why you opened the PR, but the whole point of review is to get others' opinions in case you're wrong.
Get a disinterested expert third party to review the work and say it's worth publishing, then you can complain to arxiv.