I wanted to clarify that when I mentioned "deleting" a submission attachment via the API, I meant the same thing as "clearing" an attachment. See API docs for Clearing a Submission Attachment (which is done with a DELETE request).
Under the hood, this will unlink the blob that stores the data for that attachment. The data will still exist in the DB taking up space until the daily purge cron job. The cleared submission attachment blobs should be deleted from the DB at that time. The 30 day wait period shouldn't apply because the purge script just removes any and all "unattached" blobs.
The only way some blobs may not be deleted is if they are shared, e.g. the same file with the exact same contents was somehow uploaded for two separate attachments, and were stored by the same blob. If only one of the attachments was cleared but the other remained, then the blob would not be deleted by the purge script.