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).
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).