The english grammar in these lessons, is often so informal it sounds crass. French is a beautiful language, but so is English. So please do better and utilize their elegance and use what is appropriate because even in English "put" is not.
rentrer = to return/go back in general, but when it's used transitively (with a direct object, like la voiture), it means to bring something back inside.
So here, “Mathilde a rentré la voiture” literally means “Mathilde brought the car back in” (like into the garage or driveway).
Technically, “put the car back” isn't grammatically incorrect, but it sounds a bit off in real-world usage.
We don't usually “put” cars back like they're groceries or a coffee mug ☕️.
We say:
“She parked the car back in the garage.”
“She brought the car back.”
“She pulled the car in before Paul arrived.”