Focus does not seem to erase your history in a way you may expect. Try this on Android:
- Erase your history.
- Go to HN, click any link you haven't clicked before.
- Wait for it to load.
- Erase your history. Make sure you see the notification "Your browsing history has been erased".
- Go to HN again, and see the link you've just clicked still highlighted as 'visited'.