From that audio alone, sounds like a repeat of the recent Boston event. Envoy cleared to line up and wait - on a different runway, unlike Portland or Austin - but began takeoff instead.
However the comms audio is incomplete (again!) at that critical point, making what actually happened a bit foggy for the moment. Did Envoy make an uncaptured readback? And what was it? Did Envoy mishear the "landing traffic on crossing runway" and think they had to make an no-delay takeoff because the landing aircraft was behind them on final?
Controller seemed slightly shaken (initially called Southwest as Skywest, asked Southwest if he (the controller) had made a mistake that led to the GA). But call-sign mixups do happen, and I expect the recent slew of events has left everyone a little nervous and gunshy.