From the contribution bible: "A miniseries is defined as a single-season television series telling a complete story in a pre-determined number of episodes."

This show was originally planned as an anthology series. Season 1 follows a complete narrative arc. The planned Season 2 ("Heathers: Revolution") would have told a completely unconnected story taking place in a different country and time period (the French Revolution). Given that Season 1 is the only season that exists, and given that it tells a complete story, I think it's fair to classify it as a miniseries.