In order to download the comments and rubric tags of a video, you must either be the owner of said video or have 'High Permissions' to a shared video. For videos shared with you, High Permissions give you the ability to download, duplicate, and edit a video, as well as the ability to export the comments and re-share the video.
If you need High Permissions to a video shared with you, reach out to the original uploader of the video and ask that they change the Share setting for you on their video to 'High.' The person who uploaded the video can change the permission level by clicking on the orange 'Share Video' button on the top right of the video's webpage, and then clicking on the 'pencil icon' next to the current sharing level to select the desired sharing level.