Looking at that site, if you switch between versions, it is not only the content that changes, but also the layout.
So I guess what they do is to version the rendered site, and that makes sense for several reasons.
So, a (scripted would be better) workflow for a new release could be:
hugo --destination static/CURRENT_VERSION
- Add CURRENT_VERSION to
Site.Params.versions
or something - Start working on NEXT_VERSION