From having used the traditional iron & drip method, I am going to say that this other way is not as good for getting wax deep into the base.
The board is usually at room temperature, which means that the wax solidifies when it touches it. Ironing the board not only melts the wax, but heats the board up to allow the wax to seep into crevices before it cools off and hardens again. It takes time for the base of the board to reach the appropriate temperature, especially the edges because the metal absorbs a lot of the heat.
In my opinion, this "japanese" method doesn't allow for enough contact time with the iron to heat up the board