I don't think this open letter is for Facebook. It is for the general public.

They were at one point in time [0]. Formally they asked for unlocking the iPhone, which ultimately isn't too hard given we are talking about a 3 letter agency. It was more about a general rule for backdoors.

[0]: https://en.wikipedia.org/wiki/FBI%E2%80%93Apple_encryption_d...

Apple makes a big deal about on-device security but they heavily promote backing up data to iCloud. They hold the keys to iCloud encrypted backups, and your backed up messages are happily handed over to LE when asked. That's why "going dark" isn't a problem on iMessage.