Goku first used Perfected Ultra Instinct Giant Form in the Dragon Ball Super manga chapter 66, "Moro, Consumer of Worlds", to combat the massive Earth Absorbed Moro.

Goku becoming stronger than Zeno by the end of Dragon Ball Super, but then being excited to train with Uub at the end of DBZ, wouldn’t really make much sense.

Once Goku triggers Perfected Ultra Instinct, his powers increase so much that he can rival Jiren, Moro, Gas, and Granolah, who became the strongest in the universe after a Dragon Ball wish.