POC -> fine
Actual mass exfiltration -> not fine.
https://gdpr-info.eu/art-4-gdpr/ section 12.
Under the GDPR this was a breach, pure and simple.
One thing they could potentially get in trouble for is Article 14 notification to data subjects (since they have emails), but that's not very heavily enforced. DPA would probably just tell to send the notification.
Good luck arguing that you had legitimate interest in data of people who did not consent to you having it in the first place.
The GDPR does not have a 'legitimate interest' category that includes exploiting breaches. White hat hackers are neither processors nor controllers and can never claim legitimate interest in the data.
Indeed, it would look a lot worse if you had.
my second guess was meth, third strict Constitution AIst
This is beyond silly.