In essence you are correct. The amount of energy returned by burning the hydrogen with oxygen will be equal to the amount of energy required to split the water in the first place.
But that's not usually what they are talking about when car manufacturers talk about the "efficiency" of the fuel. Usually when they refer to efficiency, they are talking about the efficiency of the burning process in the use of the fuel, which in the case of Hydrogen/Oxygen is perfectly efficient.
There is also the benefit of a combustion byproduct which is simple water so it doesn't need any special disposal processes or additional conversion processes.