The snapshot they took before the release of TBC would have been an important keystone backup to have, they wouldn’t remove the backup. Sure lets say they permanently deleted the characters from production, that’s fair to say (even though I am certain they did not remove the records completely) the backup leading from classic to tbc still exists, they have to have that one otherwise that would be a colossal miss-handling of data to not have that classic-vLast backup.
Now for the excuse that it’s too hard to bring the data back into production from a backup. Here is how easy it is in SQL:
INSERT INTO target_database.characters (id, name, …)
SELECT source.id, source.name, …
FROM source_database.characters AS source
LEFT JOIN target_database.characters AS target ON source.id = target.id
WHERE target.id IS NULL;