I don't think it is? I happened to go through my old follows and saw this was marked completed, to my surprise. I checked all the trackers and they're all listed as ongoing. Also checked in on NicoNico and they have up to 32 chapters published with "Next update date: Scheduled for June 27th." So, I've no idea where the supposed completion comes from, unless the author stated so somewhere.
Edit: Decided to do a more thorough look around real quick to be doubly sure and found the author's Twitter. It is completed with 4 volumes 😅