In #11237 @andruud's draft language for @import scope() includes the following note:

Note: While the [=style rules=] within the imported stylesheet
become [=scoped=],
they do not become [=nested style rule|nested=].
In particular,
top-level selectors are not re-interpreted as [=relative selectors=],
and the ''&'' pseudo-class maintains its non-nested behavior.

This behavior wasn't discussed is the original issue #7348

There has been some discussion on the PR, but this seems like a better place to have the full conversation and come to a resolution. Applying scope without nesting would be a departure from the behavior of @scope rules.

@romainmenke points out that the difference makes it hard for tooling to merge files, and wrap the imported sheet in a scope block. And Anders says:

We'd have to allow relative selectors top-level, but we could do that and spec that they are relative against :scope.

(Re-interpreting & is deeply incompatible with Blink's implementation of nesting, but we don't have to let that guide the discussion.)