Writing Rich Text Story doesn't add user as a collaborator

I am submitting the following story through the API via https://developers.asana.com/docs/#create-a-comment-on-a-task:

<body><a data-asana-gid=USERID data-asana-type=user />: Test Message</body>

When the comment is visible in the UI the comment is correct:

@USER: Test Message

However, the user is not added as a collaborator nor is the user notified about the comment. I understand I can use the https://developers.asana.com/docs/#add-collaborators-to-a-task to add the user as a collaborator.

However, one would think that using the API rich-text to @-mention a user would also add them as a collaborator (should follow the same logic as the UI).

Is there something I am missing or is this behavior expected?