It was published in Preprints.org, a multidisciplinary preprint server run by MDPI.

I'm not an expert here - is it correct to take anything from here or arxiv as default skeptical?

The definition of a preprint is that it isn't peer reviewed. Unless you're an expert in the field, you IMHO shouldn't be looking at preprints. Might be OK if they come recommended by multiple unaffiliated experts (i.e. kinda half reviewed), but definitely not by default.