I’m not convinced a guy who seems to have a real chip on his shoulder is real reliable about what is really mass or serial killing. No way Detroit alone has had that many in those categories.
Technically a mass murder requires at least 3 deaths, so automatically several listed are off. It also generally means random shooting in place with some kind of targeting intended, although not necessarily personal. Serials are similar, but the random targeting is spread out over time.
My guess is mostly these guys were regular thugs who randomly killed several people in the course of gang-banging. Some of them probably accidental, by dint of crossfire. Some just part of the drug wars.