> In this case, Meta explicitly promises that Muse will not read your messages even after you grant it disk permissions.

There's no reason to doubt this claim. The only person in the world who has claimed that Muse disrespects its own internal setting is the same person who claimed that he didn't grant Full Disk Access to Muse.

Ironically, Aten's own screenshot appears to show that he toggled the internal setting from "Off" to "Read only". In my own testing, it's "Off" by default, and the only way to change the internal setting is to enable Full Disk Access first.

Thus, the likeliest scenario is that Aten unthinkingly granted Full Disk Access to Muse, granted the Messages app permission, then had a change of heart, disabled Full Disk Access, and then forgot what he had done. Later, when he noticed that Muse had some of his messages, he went back and checked, and saw the FDA was disabled, forgetting that he had toggled it on and off.