You're right that it's not due to the GDPR, but existing EU law already required them to provide all user info on request. See Max Schrems' work, specifically his 2011 complaints to the Irish Data Protection Commissioner, and the subsequent Europe v. Facebook case.
The Facebook download tool has been around since 2010. It's not correct to suggest they created this in response to GDPR.
You're right that it's not due to the GDPR, but existing EU law already required them to provide all user info on request. See Max Schrems' work, specifically his 2011 complaints to the Irish Data Protection Commissioner, and the subsequent Europe v. Facebook case.
http://europe-v-facebook.org/EN/Objectives/objectives.html