It looks like JS doesn't expose equality operator which can distinguish different strings. Thus "abc" and "abc" are the same object, no matter how they are produced, even if under the hood they are separate instances.
The spec indeed goes through some trouble to ensure they are pure value-types and do not exhibit any reference-like semantics, for instance by prohibiting their use as keys of WeakMaps and WeakSets - along with numbers, booleans, nullish values, and bignums.