When he asked Muse how it got the information, it said that it read banners from incoming texts. But that's not true, either.

After doing a little digging, Aten says Muse synced 187,000 lines from his Messages database, despite Full Disk Access being off.

I'd like you to read that a second time. Meta's similarly named Messenger app didn't get ingested.

Apple's Messages did.

Unless Aten is the most popular person on the face of the planet, I sincerely doubt he'd gotten 187,000 lines of text in roughly 24 hours. What happened is that, without permission, Muse went into the app and downloaded the information.

He had not given Muse permission to access Messages, and full disk access was not enabled. It just went ahead and uploaded those texts anyway.

Muse can take "unexpected actions" indeed.