Page length is a common indicator, though it varies by conference. This paper is 3 pages, which is in most subfields of CS a typical poster-paper length (2-4 pages), with full technical/research papers being 6+ pages. But in this case it also says so on the arXiv page.
The comments section says it's to appear as a poster. I don't know how it is for the organization hosting that event, but when I was a physics student, every member of the physical society was allowed at least one publication per year. So anybody who didn't get a paper or a talk, could do a poster, and their abstract was printed in the proceedings. I don't know if it was an official rule, but it's what happened in practice. I think it was not a bad approach, as it meant that nobody could say they were denied publication of their idea.
It was also fun to visit the crackpot posters. Some of the authors went to a huge effort, including having their own books printed, and so forth. Plus, there was usually beer at the afternoon posters.
In CS typically the poster track is peer-reviewed like the main track, just with different standards that are oriented towards allowing people to present preliminary work, minor experiments, etc. However it is still supposed to be interesting and competent work, and you can definitely get rejected even with a non-crackpot poster submission if it's one of the more competitive conferences. You usually have to submit a paper for peer review, but a short one, <4 pages. At some, it may also be a consolation prize for rejected full-track papers, where work that doesn't make the cut but is still interesting is offered a space at the conference if the authors are willing to submit a cut-down version as a poster paper.