From the article…

I want to make one technical point absolutely clear because several people have misunderstood my argument:

The encryption still works. OpenAl hasn't cracked or mathematically broken iMessage encryption. An iMessage remains encrypted between endpoints. The privacy problem occurs at the endpoint.

Once the message has been decrypted on the recipient's Mac, that recipient can authorise third party software to access the readable conversation. The encryption worked exactly as designed. It simply can't protect message content from software authorised to access it before or after decryption.

That's what I mean when I say this integration undermines the purpose of end to end encryption.