When I tried Brave, it blocked website ads but gave me browser ad popups on the top right. No thanks. Stick to Firefox.
This says it's opt out, not in.
If you mean the sponsored ads on the New Tab page, then yes they are indeed opt-out. But the person who I responded found the browser pop-ups annnoying. That is a part of Brave Rewards. The New Tab page ads are not a part of Brave Rewards unless you explicitly enable that in the settings (second section of the link you mentioned).
You do not need to participate in Rewards in order to enjoy Brave. However, note that sponsored images on the New Tab Page are on by default, but can easily be disabled. (If you enable Brave Rewards, you will earn for the New Tab Page sponsored images you see.)