@mommy You can change the form and the color of the barrier. So how I look at it, is as if you are making the barrier so that it covers only your body (like tight suit) then make it outside of your body then you would have a copy of yourself. After that you would only need to change the texture and color of the barrier and voila, you have a copy of yourself.
Flying is easier, since the barrier can fly, you can stand on it to fly.
This is most likely the thought process of the author.
It isnt like it is pulled out of his ass though, if you look at Tower of God you can see the same elements shown by the light bearers with their light houses (which are simply put barriers). Those elements were used by more then one author and are kind of known by the majority of the authors and readers and thus, are mostly accepted.