Tab groups might do this since one of them is private browsing. If groups aren't isolated, implying they are by putting private browsing in there would be a huge UX fail.
I believe private browsing tabs in Safari have been isolated to some extent for a while already — if you sign into something in one tab, you aren't signed in in the other tab, suggesting separate cookies.
Wow nice! Now if there was a containers extension I could switch to Safari and away from Firefox.