Dim Sum is translated as tea lunch. It would be very unusual for a restaurant to start serving it before 10am. for one thing, the prep time related to getting the items ready to cook takes up most of the morning.
Dim Sum translates (roughly) to points of heart, but the typical way it's referred to in Cantonese is Yum Cha or "Drink Tea." It doesn't say anything about lunch, though. In other cities, I've seen it open before 10, but DC doesn't seem to care that much about Dim Sum.