I don’t think they’re the same thing. Site isolation prevents Site A to which I’m logged in to know that I’m visiting site B. Multi-container prevents Site A to which I’m logged in to know that I’m visiting same Site A in another tab without being logged in or with a different login. They’re not the same. Typical use case is being logged into work Gmail and personal Gmail.

If you’re on Firefox, take a look at the Containers extension. It helps keeping everything separated