The auto-battle only works for someone who has Great Sage. She took over.

I like that she was a little put out when he cut into her explanation. Cute gag.

What wasn't clearly shown in the anime, though it was there, but called out explicitly in the manga, is that this wasn't just Predator eating the Orc Disaster. The Orc King has green gas spilling out, devouring everything near him or in direct contact with him. That was what the Rimuru was referring to about 'your Devour versus my Predator.' It was the respective powers of two demon lords battling it out to consume the other. Parts of Rimuru were being ripped off and absorbed into the Orc King to regenerate him.

Eventually, Rimuru's will beat out the Orc King's. The orc king became a demon lord to save his people. That destruction was his sin. Rimuru promised to consume all that sin, as well as the sins of the orc king's people. That "satisfied" the orc king, and ended his hunger. With his hunger gone, the orc king stopped trying to devour Rimuru, and Rimuru consumed the orc king's body completely.

Auto-battle couldn't have run Predator that way. That took Rimuru actively controlling his regeneration while trying to eat the orc king. It was a willpower battle.