It is important to note that although this may be a result of Reddit's UI not displaying the content users posted to now-private subreddits, it remains a problem. Additionally, I agree with the author's comments in the video description, as it appears strategically unrealistic for Reddit to ask that users manually delete the content themselves.
This is particularly true when considering that many automated methods to accomplish this task will be hindered by Reddit's upcoming API pricing changes. Furthermore, Reddit has demonstrated a recurring pattern of rolling back databases using historical backups, thereby disregarding user deletion requests that were submitted prior to the database rollback.
See similar discussion of this video on Hacker News: