> ... assuming those tabs are cross-origin.
Not sure why this has to be a condition. Perhaps tabs loading pages from the same origin will both have read/write access to some shared data in the browser kernel (like the site's cookies), but they still run in separate address spaces regardless. A compromised tab won't be able to directly access another tab's RAM, as is the case with single-process browsers.
> Also assumes that a compromised tab can't go on to compromise the OS kernel.
Very true. However, my argument was that a multi-process model limits (but obviously does not eliminate) the impact of zero-days. In the single process model, the attacker could compromise any tab and have all tab state available with no additional effort. In the multi-process model, the attacker would have to compromise the right tab, compromise a different tab and trick the browser kernel to performing the requisite operations, or somehow bypass the OS's memory protection. Each of these require more work than before.