Freelancers have been kept on retainer for quite a long time. It sounds like the author just wasn't familiar with that business model. Which is fine - I don't want to be harsh on someone just because they had a knowledge gap - we all do. But there is more to being on retainer than just a monthly fee.
There still is often hourly estimation because the retainer frequently will be for a set number of hours, with additional hourly rates if you go over that limit. (Freelancers who offer unlimited hours get in trouble fast.) Companies therefore still triage their needs, to try to get as much done in the set hours without incurring too many additional hours.
There still is often hourly estimation because the retainer frequently will be for a set number of hours, with additional hourly rates if you go over that limit. (Freelancers who offer unlimited hours get in trouble fast.) Companies therefore still triage their needs, to try to get as much done in the set hours without incurring too many additional hours.