In _config.yml, I’d like to do something like this:
description: >-
some
really
long
description
bio: {{ description }}
In other words, I just want bio to point to, be an alias to, or be an exact copy of, description, so that whether I use {{ site.description }} or {{ site.bio }} in my liquid throughout the site, I get the same thing.
As you discovered, you can’t use Liquid inside _config.yml, but the YAML format has its own mechanism to repeat nodes using achors (&) and aliases (*). For example, if you add the anchor &desc to the description node, then you can aliases is under the bio node:
description: &desc >-
some
really
long
description
bio: *desc