If they're going to use per-tab containers, doesn't it make sense to have per-tab sandboxing as well, to ensure there's no data leakage?
They just barely separated the chrome process from the rendering process. All tab content is rendered in the same process for now. Eventually they will separate it further but it's not done yet.
It currently has some delay when you open a new tab, and a few quirks here and there, but it's mostly working.