To make things clear - Maldacena's universe is not like the one we actually live in! It's a model, a toy universe, which comes complete with its own physics. It's a hologram because all the physical goings-on inside it can be described by a physical theory that's only defined on the boundary. What's more, it's a universe in which the gravity/quantum conundrum has been resolved completely: the boundary theory is purely quantum, it contains no gravity, but a being living in the interior will still experience gravity. Gravity in this universe is part of the holographic illusion.