Do you think the use of OCI containers/images is a mistake/bad choice from blendOS?
No. It’s probably the best way to run packages from Arch, Debian. Ubuntu, Fedora, and others, all on the same system.
How is NixOS different?
NixOS simply doesn’t tackle that problem, so it doesn’t come with containers out of the box. If you want to run packages from other distros on NixOS, you’d probably need to manually configure the containers.
I feel like you’re under the impression that the three distros, NixSO, blendos, and Vanilla OS, have similar goals. I don’t know about Vanilla OS, but the main similarity between the other two is that they’re both non-standard in some way.
But they’re actually solving completely different problems: BlendOS wants to be a blend of different OSes, NixOS wants to have a reproducible, declarative configuration (declarative here means, you don’t list a bunch of steps to reach your system state, but instead declare what that state is).