In the API article slugs are always passed as a GET parameter, unlike in the case of browser visible URLs. This is because we don't care about having:
  • nice human readable URLs
  • ISR, at least for now
so the id= parameter is always used.
For now there is no API that returns single items: getting a single item is done simply using a filter that uniquely selects a single element, e.g.:
Maybe this will change if someday we ever de to have full vs minimized versions of API objects. But then at that point we might as well go to GraphQL.