The law covers personal data not PII and is much broader than PII that is defined under the US Office of Privacy and Open Government which has no bearing on the GDPR.
As for personal data, email addresses are covered and even user names could be considered personal data depending on if it's linkable to a person and since your account is linked to your email and linked to you user name potentially, when they have to delete personal data the have to delete all of those foreign keys in their database that says your email once got a receipt for that game you bought digitally.