When Chén Nuò was told that Celebi would return Whimsicott to him completely unharmed, to be honest, he was extremely tempted.
Since Celebi intended to train his Whimsicott, it wouldn't just be a matter of casually teaching it a few moves. Although he didn't know what would be taught, the fact that Celebi was willing to take Whimsicott on as a disciple was an astonishingly pleasant surprise!
While Celebi might not seem overwhelmingly powerful in the games, its lore in the anime established that it was indeed stronger than most Pokémon, and according to official descriptions, it was even more powerful than Mew!
Chapter 117