The data which were siphoned off contained email addresses. These data are now floating around the interwebs and can be used by anyone. There is nothing Plex can do to prevent it.
It looks to me that somehow Netflix saw fit to use email addresses which were gathered in such a disreputable way.
IMHO a very questionable action by them. It maybe worth asking them directly how they usually acquire marketing email adresses.
I have just had a Netflix spam mail that came from info@mailer.netflix.com. It also came to an email account that I created specifically for Plex and have only ever used for Plex. What’s going on here?