Using main style.css with add_editor_style

The simplest answer is: you can use whatever stylesheet you want for the editor. editor-style.css is just the default stylesheet loaded if none is passed to add_editor_style().

This will load the specified stylesheet:

add_editor_style( 'some-stylsheet.css' );

This will load editor-style.css:

add_editor_style();

Use whatever better fits your needs.

By the way, it is better practice to add the editor stylesheet using after_setup_theme action, not init, as it is something specific for the theme:

add_action( 'after_setup_theme', 'cyb_theme_setup' );
fucntion cyb_theme_setup() {
    add_editor_style();
}