Jiren's full power is said to be at Hakaishin level, if not stronger. UI Goku is said to be getting closer to Hakaishin level, but not quite there yet, and he could still defeat Kefla, though with a point-blank Kamehameha.
It wouldn't make much sense for Jiren to be hyped up as the endgame for a growing UI Galu in this arc if the opponent he already beat was superior, plus the only reaction he gave to Kefla's power was a slight twitch of his eyes. Jiren wins with high-diff at most.