The extend actually is not known but whats is known for sure is that the 800k accounts info are out.
Knowing how people tend to use the same password accross different accounts this can lead to security issues if , for example, the user's password is part of a previous hacked web site (madison ). By linking both email addresses they can gain access to his EPIC games account.
This is just an example and hackers are always been very creative to find ways to abuse system/accounts.