I think the author has the terms "opt-in" and "opt-out" reversed, which was a bit confusing. If something is "opt-in", then you have to explicitly decide to opt-in, which means that by default you won't be included.
A good example of this is organ donation. Some countries have it opt-in (i.e., you have to decide to donate), whereas in other countries it is opt-out (so you are a donor by default).
I think it describes a somewhat common scenario, where it's on by default but there's an effort made to make it seem like you went out of your way to agree to it.
Like a pre-checked box on a form about your desire to receive marketing mails.
A good example of this is organ donation. Some countries have it opt-in (i.e., you have to decide to donate), whereas in other countries it is opt-out (so you are a donor by default).