There were a lot of good guesses for what the spell was going to be - and a great many of those spells were better choices than this one - Spirit Guardians, for example, would probably wipe the fight (although it'd risk killing someone.)
So, if lower level spells might be better here, why is Konsi summoning an angel?
Well... remember that this is a downtime period, not a day of adventuring. On top of that, Konsi spent the morning preparing her spell list for dating. She doesn't have many combat spells prepared at all.
Summon Celestial isn't the most efficient combat spell, but it is versatile - when you can summon a 12ft tall flying angel, you can solve a lot of problems as well as combat - so the spell made the cut.