Returning them as an array is correct cause it can be multiple.
Doing the same, see e.g. in the client https://github.com/redaktor/widgets-preview/blob/master/src/common/activityPubUtil.tsx
Having also support for an array of IDs (as long it is an URL.protocol I can deal with) cause it is specified as being valid.
“Range: Link | Object”
see also Example 68
https://www.w3.org/TR/activitystreams-vocabulary/#dfn-attributedto
@grishka @humanetech
Additionally the array would then only need a .filter for the Actor type because
“The attributed entities might not be Actors. For instance, an object might be attributed to the completion of another activity.”