That makes sense. I think the reason I was confused is that when you first open the app, it shows the preference windows, so I tried to set the brightness below

That makes sense. I think the reason I was confused is that when you first open the app, it shows the preference windows, so I tried to set the brightness below 0 in that window. It didn't work, so I checked Allow brightness to reach zero. I then tried the keys and it didn't work due to me having checked that option. I then unchecked it while trying to figure it out... And eventually saw the screenshot on your site where the user is interacting with the icon in the menu bar) which is how I got it working.