A new beta of Mlem for Lemmy was released on 2023-07-02. Among many other improvements, it has greatly improved VoiceOver support. But there are also the following known issues:

• Large posts will prevent scrolling by using left and right gestures with VoiceOver
• Unable to navigate to a community from a post with VoiceOver

You can read all the details from their announcement post.

You can download the update from TestFlight. Remember that there are two separate TestFlight betas. The first ended when there was a change in the development team and won't be updated anymore. If you are in that beta, you need to join the new beta to get the new update.