I think there’s some confusion here. You’re talking about Multi-Account Containers, that person was talking about the Facebook Container. Both Firefox features with confusingly similar names, and honestly that’s on Firefox for naming them.
Facebook Container is similar to this TCP feature, but focused on Facebook. And of course it was a separate extension, so very opt-in. Now, Firefox has rolled it out for ALL sites by default, which is awesome and SHOULD HAVE BEEN HOW COOKIES WORKED IN THE FIRST PLACE!