I definitely could have been clearer about explaining it, but this is the key benefit pipenv/Pipfile get from moving the package names out as TOML table keys: it allows the value to be a string for a plain dependency, or an inline table for something more complicated.
That said, it would be reasonable to say that these standard fields are intended for the kind of dependency metadata that can go into a built package, and anything like editable installs will remain in tool specific formats for now.