Very interesting that their main reason for rejecting ActivityPub is GDPR. Basically, you can't delete information after its been federated out.

This seems to go to far. Give the users control over their data in your system AND give them control over how their data is sent to other systems.

Edit: In the linked issue, OpenBook CEO said that you can't delete mastodon accounts, but Mastodon got that feature a couple weeks after his post:

https://github.com/tootsuite/mastodon/pull/3728