No, the way I showed it there in the images, you can still use the supplied wall outlet, this will just bypass the step-down converter, that's why it's connected on the output side.
However be very careful when you use wall power and use a volt meter first to check, if the supplied voltage is what your camera can handle.

What is the purpose of bypassing the step-down converter?

In that instruction I use a product as a basis that already provides the right voltage for the camera when wall powered. I modify it to be able to use 12V input, that's why I use the converter. If you use the power from the "big" converter that is wall powered, it already transformed the voltage to 8V.
But like I said - the setup might vary. You should have some electronics knowledge and know how to operate a volt meter if you do stuff like this. Then you will know how to do it.
You do such modifications on your own risk - if you supply a wrong voltage to the camera, you might damage it.