To answer this question we should first ask ourselves why they need to eat and drink in the first place. Any animal, including a tortoise, needs food to provide it with energy. It needs energy to power its muscles, and keep its body temperature constant in the face of changes in the enviromental temperature.
It needs water because its body (like any animal) is to a great extent made up of water. But there is a continuous loss of water through sweating and the production of urine. Sweating contributes to cooling of the body, and urine will excrete some of the waste that is produced when the animal extracts energy from its food.
We could liken hibernation to turning down the thermostat in your house. A lower setting means that your energy bill will be less, and so it is for the animal too. What the hibernating animal does is literally turning down its body thermostat, so it uses a lot less energy in keeping its body warm. Added to this, it will retreat into a safe corner and stay there, motionless. This means no energy needs to be used for muscle movement.
When less energy is used, less has to be produced. As a result, there will be less waste that needs to be excreted, so less water will be needed for this. With a lower body temperature, sweating will also be reduced. Taken together, the animal can use its body reserves of water and fuel (fat) to survive for a long time while it is in this “energy saving mode”. Long enough to survive the period of hibernation without dying of starvation and thirst.