Your message, through several layers of indirection, relies on a security conference paper from 7 years ago[0] + the assumption that Apple haven't updated the protocol in 7 those years.
No, my message relies on the fact that people have been looking at iMessage for years, and nobody, least of all Apple, has said that the implementation changed in any way to prevent Apple from viewing the messages.
Here is another article from 2016, which shows that Apple patched iMessage to prevent attackers who don't have access to Apple's servers from reading the messages but still kept the ability to read the messages themselves. https://blog.cryptographyengineering.com/category/imessage/
Apple was aware that people knew it could decrypt iMessage messages this entire time, but Apple made no changes that would fix that. That should give you some idea of whether Apple intends to ever fix that.
Here is another article from 2016, which shows that Apple patched iMessage to prevent attackers who don't have access to Apple's servers from reading the messages but still kept the ability to read the messages themselves. https://blog.cryptographyengineering.com/category/imessage/
Apple was aware that people knew it could decrypt iMessage messages this entire time, but Apple made no changes that would fix that. That should give you some idea of whether Apple intends to ever fix that.