"Buu is really strong now!"
After eating Zamasu, Buu was so exhilarated by his increased strength that steam burst from his entire body, and he wanted to fight the other Zamasus again.
Lu Li did not stop him. Buu began fighting a Zamasu, and this time, Zamasu was clearly no match for Buu.
Buu was inherently a broken character, like chewing gum. He feared neither blows nor slashes nor impacts. The only thing that could effectively harm Buu was destructive energy blasts, but unless those blasts completely eradicated him after shattering him, Buu could gather himself back together.
On top of that, Buu had the ability to devour and absorb Zamasu, so Zamasu's immortal body meant little before him.
After they fought for a while, Lu Li called a stop to the battle and had Buu continue trying to devour Zamasu.
Lu Li said that every Zamasu Buu devoured was a benefit to humanity, and that was indeed true. Without Lu Li, Buu would have been one of the few saviors capable of eliminating Zamasu.
Lu Li had no intention of preaching humanity to a lunatic like Zamasu, who wanted to exterminate mankind. Even as Buu's feed, Buu found him disgusting.
"Buu, every time you eliminate and devour a Zamasu, I'll give you a huge cake. How about it? You can come eat here whenever you want."
Lu Li felt like some creepy uncle luring a child with candy, but he needed Buu's power. If Buu ate enough Zamasus, perhaps he could become the strongest being in the universe.
Completing his universe-level side quest was secondary. What mattered was that good-hearted Buu was fiercely loyal to his friends. When they truly faced adversity, Buu's combat strength would be a vital force in protecting humanity.
"Cake... tasty. Buu... likes it..."
Buu