To reproduce this behaviour you will need to have an existing conversation in the Messages app which has at least several dozen messages.
- Ensure that VoiceOver is enabled.
- Open the native Messages app.
- Place VoiceOver focus on to an existing conversation.
- Double-tap with 1 finger.
You should now be taken to the conversation.
Note where VoiceOver focus is placed in the thread.
It will typically be 8-12 positions above the most recent message in the thread.
It does not appear to make any difference if the conversation has unread messages or if they have all been read.