It's worth noting that not every creator or fan worries that much about whether two things are strictly in continuity with each other or not. For many, all that's necessary to call something a prequel is that it's set earlier in the characters' lives, regardless of whether its specific version of events is in continuity with what came before. This whole strictly delineated taxonomy of reboots and prequels and whatnot that some fans insist on is hardly a formal or universal set of policies handed down from on high. Heck, even the term "reboot" never really meant "launch of separate continuity" until fans started using it that way in response to the Moore Galactica. Before then, in industry parlance, it just meant a restart of a dormant franchise, regardless of whether it was a continuity or a revamp/revival of the old one. So it's worth remembering that these labels are somewhat subjective and that the usages that fans tend to favor these days aren't necessarily the same as the usages within the industry.