I think this refers to the fact that Activity Pub as a protocol was not designed with privacy in mind, but rather pubic content. So Mastodon could add some visibility features in their UI, but other ActivityPub speaking software would not necessarily understand or also add these features to their software and thus expose supposedly more private (or rather less visible) content from Mastodon users.
I have the same question, and also what it means to “have more visibility control”.
I think this refers to the fact that Activity Pub as a protocol was not designed with privacy in mind, but rather pubic content. So Mastodon could add some visibility features in their UI, but other ActivityPub speaking software would not necessarily understand or also add these features to their software and thus expose supposedly more private (or rather less visible) content from Mastodon users.
They explain it in the next sentence: