When using the GET activities API with a specific list Id in the parameter, the response will get all the specified activities from other assets as well.
This is the endpoint for this article.
Instead of using list Id (listId) as a parameter, change that to asset Ids (assetIds) in the API call.
assetIds is the Id of the primary asset for an activity.
Then, the response will narrow down the activities associated to the primary asset by looking up the Id during the call.