Huh. I'm not an astronomer. Is Instrumentation and Methods for Astrophysics a reputable journal? The paper has a doi, which I assume means it's been accepted for publication. It could certainly use a copy edit, but I assume most do. Interesting that it's a Ukraine observatory.
arXiv is one of a few pre-print repositories. medRxiv and bioXriv are two others. They're all not yet peer reviewed. From the main page of their website: "Registered users may submit articles to be announced by arXiv. There are no fees or costs for article submission. Submissions to arXiv are subject to a moderation process that classifies material as topical to the subject area and checks for scholarly value. Material is not peer-reviewed by arXiv - the contents of arXiv submissions are wholly the responsibility of the submitter and are presented “as is” without any warranty or guarantee. By hosting works and other materials on this site, arXiv, Cornell University, and their agents do not in any way convey implied approval of the assumptions, methods, results, or conclusions of the work."