I'm doing something wrong?
The "select all with a window" method you suggested does not select the Draft feature, and that's why the mirror appeared non-symmetric.
You should use the Bodies to Mirror option instead of the Features to Mirror option, and just single click the body in the graphics area, or from the Solid Bodies folder in the tree.
The object looks exactly as expected to me.
Can you attach your *.sldprt file here as you go through the tutorial?
Was neccesary to mark " Matriz de geometria" inside "Simetria-Option"
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