It's not worded super clearly, but they mean that the rope is being tied diagonally from the top of the building to a point 30 feet away from the building's bas

It's not worded super clearly, but they mean that the rope is being tied diagonally from the top of the building to a point 30 feet away from the building's base (horizontally). Like you, I initially interpreted the question to mean that the building was 30 feet above ground level, which left me a bit confused.