You're kinda right. The person you were replying to was talking about container ships that operate on liner services, and indeed rotor (or other, there are more than one kind) sails would be impractical.

However, the vast majority of ships are not RoRos. In fact, they make up a tiny percentage of the global fleet. These kinds are sails are designed for bulk carriers and tankers which, along with container ships, make up the majority of the global fleet. If you want I can post the actual numbers (I am in the shipping and oil industry and have access to the proprietary sources that track this stuff).

The EU is making a massive push, mostly through ETS, to curb ship emissions by a signifiant amount, 40% less starting next year, and going to 50% then 70% the following two years. So sails in various forms are finally becoming a viable commercial (partial) solution.