The word "sandbox" is a bad choice -- "isolation and resource limiting" might have been a better term to use, but the idea that containerization does not sandbox at all is not a fair characterization.
It's not a good sandbox, but if we are pedantic about the definition of a sandbox, it fits, especially when we think of the benefits of namespacing (effectively removing access to resources like networks, filesystems, etc).
gVisor is a more focused on sandboxing processes specifically, so it's relevant but gVisor is not relevant to the wider discussion about a packaging format -- unless you're suggesting to run gvisor'd processes instead of containerized ones and containerization is still beneficial in that scenario.
simply not true: https://github.com/google/gvisor#why-does-gvisor-exist