In this sentence "quedarse dormido" is translated as "oversleep" because in Spanish this is the most direct way to express this, so the verbal structure quedarse dormido/-a also means "to not wake up on time and stay in bed sleeping". The lesson on dormir/dormirse is not reflecting this possible meaning because it's a bit more specific but I will add a note to the lesson.
I hope this helped clarify it.
Gracias y saludos
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