Not necessarily. While "downstairs" can sometimes be used to refer to a cellar, it typically refers to the floor of a building that is below the main level, and may include basements, sub-basements, and other below-ground levels in addition to cellars.
A cellar, on the other hand, specifically refers to a room or set of rooms below the ground level of a building that is primarily used for storage of food, wine, or other goods.
So while "downstairs" can sometimes be used to refer to a cellar, it is not necessarily synonymous with it, as it can refer to other below-ground levels as well.