They were a thing, on mainframes, micro-computers, and a few selected UNIXes like the (shortly lived) Tru64 and HP-UX.

My introduction to containers was HP-UX Vaults.

Aye, BSD jails have been around for a while, and the chroot jail has been a concept since I've been using *nix OS's.

But there wouldn't have been the hardware, resources, or depth of support available for the end user in the 2000s. There are still non-technical folks I deal w/ at work that don't get what a VM is...

they were not a thing that commodity hardware, but most importantly the target user, was really prepared to run at the time

same as virtual machines were a thing in the early 70s and client-server in the 80s, but not for the target market despite the industry thought so for some time

It depends on the definition of target user/market. For IT professionals definitly yes.

that wasn't remotely the target of Active Desktop

Indeed that was for Electron like applications, and not at all matter of discussion.