Why are people so upset about Mary Sue? "Joining a war because one of my family members got killed by the opposite side" is a perfectly good reason for a 1X year teen to enlist. She's just a kid, of course that's what she would do.
So this isn't mentioned in the chapter: but they get sent back to the time when they were summoned. So it's one of those "it seemed like a year on the other world but it only took a second on Earth" type of deals. And the summoned hero does get an entire year of luxurious treatment. Also...