They're not encrypted, they're just hashes  mapped to users in a different table.
You can decipher the username by joining the user_mapping table with whatever table you need the username from. Here's an example query for the content_label table:
SELECT * FROM confluence.content_label JOIN confluence.user_mapping ON owner=user_key;
I suspect it's done this way to preserve usernames when a user is removed from Confluence.
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