No, malicious or buggy software running on a remote instance absolutely can ignore followers-only and DM restrictions. ActivityPub doesn't offer any guarantees on these fronts. That makes it unsuitable for anything confidential, and it's a good question whether most users realize it.
Then again when infosec and a bunch of other instances accidentally explosed all media in DMs to the world, and when kolektiva accidentaly shared all followers-only and DMs with the FBI, people weren't actually particularly angry about it, so maybe expectations are set correctly,.
