SmartOS constructed a container-like environment using LX-branded zones, they didn't create an in-kernel equivalent to Linux's namespaces which it then nested in a zone. You're probably thinking of the KVM port to Solaris/illumos, which does run in a zone internally to provide additional protection.

While LX-branded zones were a really cool tech demo, maintaining compatibility with Linux long-term would be incredibly painful and you're bound to find all sorts of horrific bugs in production. I believe that Oxide uses KVM to run their Linux guests.

Linux has always supported nested namespaces and you can run Docker containers inside LXC (or Incus) fairly easily. Note that while it does add some additional protection (in particular, it transparently adds user namespaces which is a critical security feature most people still do not enable in Docker) it is still the same technology as containers and so kernel bugs still pose a similar risk.