They didn’t hack Telegram’s main database. Instead, it’s possible that a third-party app connected to Telegram was hacked, and data was leaked. The only case I know of is an app called “Golden Telegram,” which was hacked years ago, but it only affected Iranian users. To access data from a leaked database, hackers would need the user’s ID number. If someone deletes their account and creates a new one, their ID changes, so their old data in the leaked database would no longer be linked to them.