I think they might have read this thread, where the Discourse OP is using Home Manager to declare podman containers because the NixOS module just doesn't support doing that. And flakes ended up in the question because a lot of other examples online just assume the reader is using flakes.
I'm not a podman user so I'm not sure how much work it would be to not use Home Manager here, it feels like it shouldn't be much more annoying than what the discourse OP is already doing, but I haven't tried.