you know, this is one of the few times i'd actually call tanya close to evil. abusing her age and using that overly childish tone of voice to make it so the enemy won't take her warnings seriously is rather nefarious. it's one of my favorite scenes in the anime. but as for why she didn't finish the job; remember tanya is at her core a being driven by logic and laws, who fiercely follows the rules laid out to her. she has to follow the rules dictated by their world's war treaties and she already stretched the rules as far as she could with the attack itself. any more and she would have stepped into illegal territory, which she'd be strictly opposed to. again, tanya is more complex than most give her credit for, it's just easy to forget about it.
anyways, some more fun details in this episode lost to the cutting board.
tanya and her squad use specialized equipment unique to them. tanya's magic orb of course is her special quad core magic orb she received from being x/one of their angels and the rest of her squad uses a reverse engineered dual core system, which, while weaker than tanya's quad core, is still far superior to the single core orbs used by anyone else, giving them a rather significant battle advantage.
it's also important to re-emphasize that in this world aerial combat is still in its infancy. part of the reason why tanya's strategy worked out as well as it did, was because tanya's part of the very select few in this world who have a decent understanding of aerial combat, thanks to her knowledge from our world. as mentioned in the episode, most commanders are still stuck in a 2D way of thinking, so they treated the battle more like a traditional cavalary attack, rather than the aerial assault it was, which left them wide open. fortifying your defence by huddling together may work if the enemy is charging you from the front, but is the exact wrong thing to do when they're attacking you from above. it's the same reason the didn't hide their command post, as they just couldn't imagine some just flying in and bombing them from the air. i'm sure they'll catch up sooner or later.