(Also, getting in the weeds, the "mostly unsandboxed" refers to that MSIX installs will do a few things to insure at all costs clean uninstall, including system folder virtualization/redirection, registry virtualization/redirection, and similar techniques. Windows and MSI has been doing a lot of this sort of thing under-the-hood silently since XP as a part of compatibility work, MSIX just does it more obviously/automatically/always.)