I’m not entirely convinced, but if you want to marry the two theories, here’s how I’d do it:
A really long time ago, humans from an old civilization come across pools of imulsion on the surface, many years before its discovery during the Era of Silence. Unaware of its true nature, a number of humans become exposed to its fumes and become the first Sires. The uninfected become fearful, persecuting the infected and driving them away. Their offspring, the first Kantus, become attuned to imulsion and soon learn about the existence of the Riftworms.
Viewing them as divine, the Kantus eventually make their way down into the hollows, establishing enclaves and temples in reverence to their slumbering gods hoping to one day discover their resting places. The centuries pass without much development; while they are largely infertile, the Kantus live very long lives and thus feel little urgency to procreate.
This changes when the humans rediscover imulsion and the later events surrounding New Hope. Unable to find a cure for lambency, the reaming scientists travel into the hollows with the new Sires and Locust for safety. It is here they make contact with the Kantus. They recognize the drones of sharing the same origins, but the existing leadership at the time viewed them as inferior and so attempted to force them into submission. Armed with superior numbers and the knowledge provided by the scientists, the New Hope Locust force the Kantus into an uneasy truce.
The New Hope Locust victory over their Ancient counterparts establishes the current hierarchy: the Kantus are largely free to pursue their agendas and have authority over most drones, but their power is kept in check by an Uzil—who is always of drone lineage and has equal power with the Ketor—and all are ultimately subservient to a human monarch.