The diagram is fine as long as you realize the only way to turn off the light is to have BOTH the wall switch and the Sonoff turned off. Having voltage on the output of the Sonoff basic while it is switched off is not a problem. It uses a relay so the contacts are open just like they are in your manual switch.
In my opinion it is ok, but you do not need to give the light an extra Neutral wire as it has already been connected to it. Look at my drawing, its simple.