Uh.
The thought that if the Summoned Hero already understands spells, she could just repeat them back, well, that thought should have occurred to him, but it's sort of large as thoughts go.
"No," Haroun says numbly. "No reason. I mean you have to know what spells mean for them to work right, but you apparently know that as soon as you hear the spell, so sure, why not, haha. You hear a spell and repeat it. Why not? That is a totally reasonable way for things to work. Try this one -"
Bella hears: "Light spirits, I offer a tenth of mana to create a flash of radiance." It's only seven syllables, and after Haroun says it there's a flash like a photographer's bulb going off, causing one of the horses to flicker its ears.