There are two different actions

• ban from site
• purge a user

The documentation explains the difference

Ban from site Completely bans the account, so it cannot log in or interact at all. There is also an option to remove all existing posts.

Purge user Completely delete the user, including all posts and uploaded media. Use with caution.

https://join-lemmy.org/docs/users/04-moderation.html?highlight=purge#how-to-moderate

I think the Lemmy user you are referring to above got banned, and their posts got removed as the sub-action, but they did not get purge.

Your account got purged on the Piefed instance you linked above.

Now obviously as you can guess it's difficult to find an example of a purged account on a Lemmy instance as there's no filter for that kind of actions in the modlog.